It’s a difference in feel caused by how the notes in a pattern are played back relative to the grid they’re laid out on.
Compare https://open.spotify.com/track/0BrDMbYgKYienYWKOO6jXP to https://open.spotify.com/track/37BTh5g05cxBIRYMbw8g2T
The kick and snare are doing (almost) the same thing, but where the hi-hats fall makes all the difference (although strictly speaking there’s more than just just swing going on here!
).
On most drum machines the default swing value is 50%, meaning that all of the 16th notes in your pattern fall exactly halfway between the 16th notes on either side of them. Changing the swing value will change the position of some notes relative to the notes on either side of them - they’ll slide in time to a little after the grid lines.
In a 16 step pattern where each step represents a 16th note, then the affected steps will be - as @alanza says - the even numbered steps.
In “most uses” yes, it’s a global parameter insofar as it’s applied to all of your parts, but seeing as the dominant voices (i.e. kick & snare) in a pattern are generally falling on quarter notes (i.e. steps 1, 5, 9, 13 - i.e NOT the even numbered steps) then they’re not affected.
If you lay out this pattern:
x—x---x—x--- - kick
—x-------x---- - snare
xxxxxxxxxxxxxxxx - hats
and then change the swing for that pattern, I guarantee that by time you hit 57%, you’ll be swaying in a far looser manner, despite the playback of the kick and snare being unchanged, and still falling exactly on the grid.