As with any moon, its gravitational field raises a (very tiny) tidal bulge on its parent planet.
Since Phobos lies closer to Mars than a synchronous orbit, Phobos orbits faster than Mars spins.
Because of friction, Mars' tidal bulge does not quite keep up with Phobos, leading to an effective drag on the moon.
As a result, Phobos is spiraling in towards Mars, and at its current rate (1.8 m/century), will hit the planet in about 50 million years.
But at some point it will reach the Roche limit, which we can calculate to be