A repertoire involves decisions like this:
As White, I will play 1 e4.
I will react to 1 e4 e5 with 2 Nf3.
I will react to 1 e4 c5 with 2 Nc3.
As Black, I will react to 1 e4 with 1...e5.
Etc.
Decisions like this can reduce what one needs to understand. For example, if a player makes the above decisions, there is no need to learn about 1 e4 c5 2 c3.
An opening repertoire is just one section of the game tree. It ignores all the positions you never reach.
So for example: if you tell yourself that you're always going to play the sicilian when faced with 1. e4, then there's no need for you to learn how to play the French, the Caro-Kann or the Scandinavian as black, since they're not part of your repertoire with the black pieces.
The benefit of this is that it makes your life 100x easier in regards to not having to know every single possible opening and variation. You only learn the things that are relevant to you.
The disadvantage is that you don't get to explore everything. You're focusing only on the positions that you'll encounter, rather than the positions that you could potentially encounter if you mixed things up more.
There are intermediate possibilities where one incorporates some variety into one's play while still avoiding many lines.
Memorizing opening lines is of marginal utility anyway unless you are already a very strong player.
Could someone please clarify what is meant by building an opening repertoire?
I have a very good application called kings cross which helps to memorize openings. But I don’t know how to benefit from master games and opening explorer apps. Your help is appreciated. 🌹
Thanks in advance