Love locks are, and I quote from Wikipedia, "a custom by which padlocks are affixed to a fence, gate, bridge or similar public fixture by sweethearts to symbolize their everlasting love". A minor but crucial detail is omitted in the quote: you should also throw away the key. Since love is everlasting, you'll never need it again.
I'm sure you've heard about the custom or seen it yourself. Perhaps even partaken in it? It has existed in Stockholm for at least a couple of years, and I recently discovered it in my hometown as well.
What do you think about it? Being a romantic person myself, I think it's a beautiful expression.