A goo trail is the slime that certain enemies leave on the floor behind them, which damages you if you step on it.
Which enemies leave one
Slime Pits is described as a world of goo-trail-producing enemies, including the Slime Wizard and the Slime Skeleton. The Slime King also leaves a trail as it moves toward you while summoning slimes.
Resisting it
The Slime Ring carries a Slime resist effect that negates any damage from stepping on slime puddles.
Note that this does not protect you everywhere: the Blood goo left by Blood Rooms enemies is explicitly not affected by it.
Compiled from what this wiki already says on Slime Pits, Slime King, Slime Ring, Blood Rooms. No new facts were added.