In Texas Hold’em and other poker variants like Omaha, community cards are cards dealt in the middle of the table, face up, and all players can use them with their hole cards to make the best possible five-card hand.
In Texas Hold’em, players are allowed to use community cards however they like, i.e. combine them with both or just one of their two hole cards, or they can even use just the five community cards if that gives them best hand they can make.
In Omaha, players can only use three community cards together with exactly two of their hole cards. No other options are available.
Community cards are dealt on three separate “streets.” On the flop, which is the first street, three community cards are dealt. After that, one community card is dealt on the turn and the river, for a total of five cards.