Loop is an informal game term. It refers to at least two cards whose abilities enable each other, allowing for two or more actions to occur infinitely. A classic loop, from the Fruit Loops deck, is Steadfast Champion (7U49) and Treebeard, Keeper of the Watchwood (10R18) with Gandalf in play. You play Steadfast Champion (7U49) , discard it to heal Treebeard, exert him twice to play it again, going back to step one with a minion and shadow condition discarded.
This TLHH thread has a detailed explanation of the most popular loops in Open: https://lotrtcgwiki.com/forums/index.php?topic=237.0