Temples are holy places of worship, each dedicated to a specific Deity.

Travelers can visit them to obtain general healing and restorative cures, increase reputation by donating Gold Piece
Currency in the Baldur's Gate series., and purchase some limited items such as potions, healing/protective scrolls, and even the occasional magical and/or non-magical item(s).

Note that there are several NPC's who are encountered while adventuring that aren't associated with a fixed temple location, but still offer all of the healing/donation/store options.