You can bounce grenades off solid surfaces up to three times before they explode, rather than once. You don’t need to see the surfaces, but you do need to have a general idea of where they are. If the grenade hits a solid surface you didn’t expect to be there, it explodes. For example, if you toss a grenade down a hall you can’t see completely, the grenade will bounce off the hall’s walls and doors but would explode if it hit a chair or a janitor’s cart.