Safe Houses are locations established up by the Joint Task Force (JTF) to provide a safe haven for fellow comrades and Division agents. They are generally hidden — a vacant movie theater, underground in the sewers, abandoned subway tunnels, etc etc — and provide a temporary reprieve (and resupply) from the atrocities of New York City for players. There is at least one Safe House per district, each secretly marked (quite blatantly, I'd say, how haven't the enemy factions caught on yet?) with the symbol(s) below:
When discovering a Safe House, you will be rewarded with a small amount of XP and Fast/Quick Travel will be unlocked for that specific location, for ease of access.
Safe Houses also present players with a universal Open World hub, where they can meet and join up with other agents. Each provides a Matchmaking option to more easily pair with other agents of your level and/or in the vicinity. Every location has a vendor (either weapon or gear) that provides the opportunity to upgrade your equipment and a Stash Crate (Player Stash) so you can swap out and/or store all your sweet Loot!
Furthermore, each Safe House will have an Intel Officer you will want to talk to and a Situations Board you'll want to populate. This will allow players to unlock the various Encounters and Side Missions available in that district.
There are similar hideaways in the Dark Zone, but they are referenced as Safe Rooms. In general, though, they serve the same purpose. A location away from your Base of Operations where you have access to your Stash, vendors, restock crates, and a moment to catch your breath after dealing with the vile scum roaming the streets.
Have you visited and unlocked all of the Safe Houses in the Open World of The Division? Have you ever found decent weapons or gear provided by the vendors in them? Ever use a Safe House's Matchmaking system? Have you visited all of the Safe Rooms in The Dark Zone?