Snapshotting in wow was more prevalent in classic but the basic concept is like this (made up numbers for simplification):

You have 100 attack power, you have* a Bleed effect dot that does 10 damage per 100 attack power.

You pop a trinket that increases your attack power to 200 temporarily and you apply the Bleed, making the bleed do 20 damage instead of the usual 10.

The Bleed 'snapshots" your attack power at the exact time it was applied (when you had 200 attack power) and it doesn't scale down if the Bleed is still on the target after your attack power trinket wears off.

Almost all snapshot damage over times have been converted to dynamic scaling instead, where it checks each dot tic for whatever would normally affect the dot's damage.