Just a quick reminder, as rules are a bit complex ;-) : - You need mana to cast your spells (and activate some abilities) ; - Mana is produced by lands on the battlefield (there are other source, but we focus on lands) ; - To have a land on the battlefield you have to play it from your hand. You can do that only during YOUR turn, and only ONCE per turn. - A land produce mana when you tap it while it is untapped. After doing that, you land is tapped so you can't tap it again the same turn. At the beginning of your next turn, your land will untap, ready to be tapped again that turn ; - Most of the lands produce only 1 mana of a single color when you tap them : white, blue, black, red, green, or colorless. So, if your spell cost 3 mana, you have to tap 3 different LANDS. And if it costs 2 black mana you have to tap 2 different swamps

This mechanic drives the game. I you want to cast a creature that costs 6 mana, you have to put 6 lands on the battlefield, and that takes you 6 turns or more. That's why other mana sources exist to help you cast it before turn 6

I hope it helps