The force field grid was an enormous force field used by Q to trap the USS Enterprise-D in 2364, during its first mission to Farpoint Station. This phenomenon appeared in the form of an energy grid. Q used a smaller version of the grid to deter security attempting to enter the bridge during his first appearance there.
Attempting to escape the grid, the Enterprise-D found that it had morphed into a kind of fireball, launched in pursuit of the starship and quickly overtaking their best speed. In a snap decision, Captain Picard ordered emergency saucer separation, despite the extreme dangers of separating at high warp velocity. Firing a volley of photon torpedoes to blind their pursuer, the Enterprise-D evacuated its civilian population to the saucer section and separated the ship, sending the saucer off to safety while the stardrive section turned about, becoming captured by the grid until Q eventually released them. (TNG: "Encounter at Farpoint")
On a relief mission to the Sigma III system, the Enterprise-D again was halted by the grid so that Q could profess to the crew the Continuum's interest in them, Commander Riker in particular. After being given the powers of the Q, Riker displayed his new abilities to the captain when, in a fit of rage as the alien creatures Q had pitted against them impaled Worf and Wesley Crusher with their bayonets, he summoned up the force field grid to block the creatures. With the crew returned to their restored ship and Q sent off to be punished by the Continuum after failing to convince Riker to join them, the grid was never seen again. (TNG: "Hide and Q")