The main pot is the center pot to which all active players contribute with their bets and which all active players are eligible for.
If one of the players goes all-in and two players that still have money continue to bet, they create a side pot for which only they are eligible. The first player is only eligible for the main pot.
For example, the winner of a side pot may not be the winner of the main pot. You may beat the opponent that created the side pot with you but have a weaker hand than the player who went all-in in the main pot.