Here's a link to their main site: https://steemmonsters.com?ref=artbyclark
Once you get there, scroll down and click on the yellow how to play button. Clicking that will take you to a page that explains the game to you. Other than that, I would suggest buying a starter set. That way, you'll be able to build a team and battle. Once you actually see the battle with your own eye's, you'll have a better understanding of how everything works.
A player should consider buying packs because, it puts you in a better position to earn more free packs from ranking tears and daily rewards. A player also has the chance of pulling extremely rare cards from packs, and has the ability to sell them on the market for a profit. I've personally sold dozens of cards now and I've made more than a few hundred doing so. Other players can and are currently doing it as well.
Another reason a player should consider buying packs is because it puts you in a position to where you can level up your cards. Leveling up your cards is important because higher level cards equal stronger monsters. One level can be the difference between you winning or losing a battle.
Simply put, you choose a certain number of cards and they make up a team. Your team battles another players team. You want to win as many battles as possible. Each monster and summoner has stats and some have abilities.
The site goes more in depth with these things. You'll see once you go there. Look around steemit for people posting tips about how to play. That's pretty much all I can tell you. I hope this helps.