It is a coherent (not random) set of openings for White and then for Black against 1.e4 and 1.d4.
For example, when I say not random, the openings you play should have something in common from a stylistic standpoint.
For example, the following sets of openings go well together from Black's standpoint:
French and King's Indian - Both often lead to a blocked center with play on the wings. You always attack in the direction that the blocked pawns point, and so in the French, White attacks Kingside and Black Queenside. In the Classical King's Indian, it is just the opposite, but still follows the same strategy of the blocked center.
French and Stonewall/Classical Dutch or Nimzo-Indian - This allows you to answer 1.d4 with 1...e6, avoiding the Trompowsky and all of them start out as light square defenses, and specifically trying to maintain control of e4. All 3 are dynamic defenses, but with them being light-square systems, they tend to be safer than say, the Modern Benoni and other dark square systems as the Black King's starts on and castles into a light square, and so king safety in dark square systems may require an extra king move (...Kh8), which can cost valuable time, and so risk is higher.
1...e5 and Nimzo-Indian - If you hate fixed pawn structures, often these defenses lead to more fluid pawns, room to maneuver pieces, and you are OK with having to know many pawn structures as your focus here is more on piece play.
Alekhine and Leningrad Dutch - I know someone that plays this pair of openings over the board. He hates closed positions at all cost, and has no concern for King safety. He also plays the Vienna Gambit as White (1.e4 e5 2.Nc3 Nc6 3.f4 and often does not castle).
Berlin and Orthodox QGD - Two very slow and strategic defenses.
This cohesion is important and when you combine that with your White game, you have what is called your opening repertoire. Someone that plays erratically as Black is likely to play 1.e4 with erratic lines like the Morra Gambit or Grand Prix Attack, the Kings Gambit or Vienna/Vienna Gambit, 3.Nc3 against the French, Advance Caro with 4.h4, etc. Someone that plays say, French and KID or Dutch might play more closed lines with e4 (Closed Sicilian, Advance French, Slow Italian, etc) or 1.d4/2.c4. Those that play the Berlin and QGD may go for lines as White like the Colle and London Systems.
What you don't want to do is just pick random openings out of a hat that really don't go together, such as:
Berlin and Leningrad Dutch
Sicilian Dragon and Lasker QGD
Pirc and King's Indian
That last one may raise a few eyebrows as people will tell you the Pirc and KID are the same. That is so far from the truth. Black's position after 5 moves is the same, but White's isn't, and chess is played with 32 pieces, not 16. In the Pirc, with c4 not played, the d4-pawb is NOT weak. The e4-pawn is only guarded by the c3-Knight. With c4 not played, that knight can often be easily attacked by b7-b5-b4. Deflect the Knight and the e4-pawn could be hanging. Black is out to attack e4 in the Pirc. In the Kings Indian Defense, White has played c2-c4. His control of the center is more dominant. With c2-c4 played, the d4-pawn, and d4-square, has become weak for White. Also, with c4 played, b7-b5-b4 is almost impossible as White can play c4xb5. So there is no dislodge of the Knight on c3. So the attack is not on e4, but rather, d4 in the Kings Indian, in similarity to the French. This is why the French is more in line with the Kings Indian than the Pirc is. Many GMs that played the French also played the Kings Indian, such as Wolfgang Uhlmann and Igor Glek, just to name a couple.
This is why in the beginning, it is important to learn opening concepts, not opening theory, and at around 1600, you need to start thinking about what openings suit you best, and study and UNDERSTAND - DO NOT MEMORIZE - said openings. I would know this because I have tried to play every opening at some point or another. Some of them I merely memorized. Others I have made sense out of. Now, being older, I stick only to openings I understand, not ones I merely memorized, but cannot make sense out of the position after the opening. This is why I am capable of playing openings like the French, Taimanov Sicilian, Kings Indian, Nimzo-Indian, Dutch, QGD, etc, and NOT capable of playing the Dragon, Najdorf, Grunfeld, or main line Modern Benoni (the few lines that can come from the Kings Indian, like the 4 pawns, I am fine with, but the Classical lines and modern lines with 7.h3, uhm, NO!).
You have to be honest with yourself and play what makes sense to you, even if it results in fewer flashy wins with cool tactics. Maybe your wins all come from grinding out small advantages.
What does an opening repertoire look like? What's yours? Post your openings here.