It's a funny phrase: loanwords. Linguistically, this refers to when the speakers of one language "borrow" a word from another language to express a concept, either because they're also taking the concept from that language/culture (like the really cool German word Sprachgefühl, which really has to be more explained than translated), or because their own native word has changed in meaning or connotation.
Occasionally, though, languages that have borrowed a word will pay it back, with interest. The old Germanic languages were rich with vocabulary relating to the lively mythology of elves, trolls, dwarves, giants, etc. So when the Franks, a Germanic tribe, stopped speaking their native Germanic tongue somewhere around the 7th or 8th century (I think) in favor of a version of Latin that became Old French, they found they still needed to keep, or come back and borrow, some Germanic words.
Take "troll", for example. Troll was the old Norse, German, Saxon, English, Franconian and Frisian word for "troll"; it really hasn't changed much, in either spelling or meaning. We tend to know what a troll is from fairytales; other cultures usually have to learn the concept along with the word (though often they have something somewhat analogous). Anyway, so Old French borrows or preserves the word troll, and even forms a verb troller from it, meaning "to act like a troll", i.e., "to wander about".
Here's where the transaction gets interesting. The French, having taken the word and done something new with it, now give this new word back to a Germanic tribe, the English. Paid in full, with interest.
But wait: there's more! These ginsu knives--sorry, wrong channel. Now the English look at their repaid word and say, "We already have our own Germanic word that means to wander: 'wander'! What shall we use this one for?" In the end, there were two answers: music and fishing. "I say," someone says, "you know how we do that singing-in-parts thingy? I rather imagine it's as if the melody wanders from person to person."
So it is that we have the verb "to troll" meaning to sing in rounds; as in "Troll the ancient Yuletide carol!" (from "Deck the Halls"). And in fishing, it means to trail a line and move the boat around, still a variation on the "wandering" theme.
As a result, the English vocabulary is that much richer. So, er, don't hide your words under a bushel basket; invest them wisely. In Franks.