F5 Bombing is the act of holding down or rapidly pressing the F5 key in order to refresh the page, and therefore make the view count 'explode,' which is where the "bomb" part comes in.
Often used in forums where the threads have view count (such as 4Chan or Reddit, more commonly Roblox).
Used mostly by trolls, in order to make the view count seem substantially higher than it should be.
It's also a parody of the word F-bomb, which is basically the non-swear version of "fuck."
A: Hey dude, why does this thread have a lot of views compared to the other threads?
B: Someone must've F5 bombed this thread.
A: What a troll."
B: Same.