I always wondered what's this all about.
Started in Paris I believe (might be wrong, that was the first place I saw them). Young love places a lock on a railing - typically on a bridge signifying their unbreakable affection for one another and throw the key away. The problem is when a few thousand have been put up and it causes structural issues on the railings as we are seeing in Paris now. They are even getting them engraved now with their names.