It means that although it's written as straight 1/8 notes, they're to be swung as 1/8 note triplets. This is actually a common way of writing for drum notation to save writing the number 3 with a bracket above all the way through.
A complete 1/8 note triplet is written as three 1/8 notes joined together with a number 3 written above signifying that it's a triplet.
What you see there is an 1/8 note triplet with a number 3 above but with the middle note of the triplet removed as you're only meant to play the first and last notes of the triplet.. So instead of them writing '1/8 note + 1/8 note rest + 1/8 note', they've written a 1/4 note + 1/8 note. The 1/4 note represents the same as an 1/8 note plus 1/8 note rest. Basically, it's turned a straight groove into a shuffle. Hope that helps?