"Bombing" is a slang term that means doing very badly in a performance.
When someone like a speaker or a comedian "bombs", it means that they're not entertaining and the audience does not enjoy their speech or performance. You can either say that the person "bombed" or that part of their performance "bombed".
I totally bombed.
I had a few jokes to start off the presentation with. They completely bombed.
It's common to use words like "completely" and "totally" before "bombed".
Things like movies and products can also "bomb". In this case, "bombing" means not selling well.