Modern is a format that lands in a sweet spot between overpowered and underpowered, and because of that, it can be very difficult to wrap your head around. Hopefully this article helps you understand it more.
Im going to start off with the basic stuff. "what is a format?", and "what is modern?". Starting with the first one, a format is a way to play magic. Formats make sure that someone who has been collecting cards for years cant put all their best cards together in a 60 card deck and obliterate someone who just started. Think of a format as a box, Inside each box (format) are certain sets. Those sets dictate what you can and cannot put in your deck. For example if your playing standard (this is a type of format) you can only play with cards from sets allowed in the format. In addition to this each set has a ban list, a list of cards that have been banned from play in that format (some more powerful formats have a restricted list, meaning you can play restricted cards but can only put one of each restricted card in your deck). Modern is a format with all cards from eighth edition (a very old set from 2003) and newer. Generally, the more cards in a format, the more consistent and powerful the average deck is. This is because with more sets, deck builders have more cards to choose from to meet their decks goal / way to win the game (Examples include creatures, a specific combo, draining your opponent of resources). Because modern has such a wide selection of cards, it is a fairly powerful format.
Great, now that you understand what modern is, lets get into how it works.
The best decks are built to accomplish a goal, and are able to do it consistently. Usually in modern your deck should win (or set up a winning position) by turn 4-5, so if a deck cant find the cards it needs to win fast, your opponent will beat you to the finish line. Because of this, your deck should only contain cards that help your deck meet its goal or cards that help you find the cards needed for your deck to meet its goal. This is very important, and often overlooked by players new to the format. When building a modern deck, I like to start with a few cards that work together to achieve a common goal, then I work to build those cards a shell (add cards to the deck that work well with those cards). Your shell should do one or all of these things:
make your goal more achievable or happen more often
protect your goal from the other deck, so your opponent cant stop / delay your goal
if your goal doesn't straight up win you the game, make it better
you might have one more question "how do I tell if a card is good in modern?". Well, I can start to explain, but this is the type of thing that comes with experience. When evaluating a card for modern, check to see: "how much mana does it cost?" (is that amount obtainable?), "is it good against other popular decks?", "are there many other cards that work well with it?", and "if i play this on turn [insert converted mana cost here], how many turns will it take for this card to be effective?"
I hope this helped you get a better grasp of what modern is, if you have any more questions please comment them below and i'll (eventually) respond.