An upcard is a card that is dealt face up so that all players in the game can see it. Generally, the term is applied only to cards that are dealt face up to an individual player, but it can be used to describe the fact that community cards are always dealt face up, as well.
Some forms of poker do not have upcards (such as five-card draw for example), but most do, since it allows for much more use of strategy. Being able to observe upcards allows you as a player to consider possible hands that your opponents may have, and gauge the strength of your current made hand or draw against them. It also gives you information about how many and which cards are remaining in the undealt deck, since any cards visible as upcards obviously cannot remain in the deck.
Upcards are intended for all players to see, so it is considered bad manners (and indeed, is often against the rules) for you to obscure your upcards in an effort to conceal them from the other players.