If you're talking about people who write that in their name selling thread, it means that once a name has been freed up, anyone can take the name. For example, if someone is selling the name 'X':
The name 'X' gets changed to 'Gf X', so 'X' is the previous name.
The new name 'User X' gets changed to 'Gf X2', so the name 'X' is free for someone to take.
The person who is buying 'X' does not manage to get the name, but someone else manages to take it, so the name 'X' is said to have been sniped.
Sniping a name is basically when someone manages to take (snipe) a rare name whilst someone is selling/transferring it.