Tidal forces are the effect of a massive body
gravitationally affecting another massive body. A
classic example is the
Moon's effect on
Earth. More
specifically, the gravity of the
Moon "tugs" on the
Earth's oceans causing them to swell. These are
called tides.
The effect of tides is not
restricted to
Earth. The intense gravity of
Jupiter also causes "tidal flexing" of the moons of
Jupiter.