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plug your your guitar into your amp, turn on amp, turn volume to 10, you hear a howling noise something like you would make if you tryed to step over an electric fence and found out to late that you weren't tall enough... if you only have an accoustic guitar, forget you ever read this post.......

so, if there's a guitarist who sometimes uses feedback during his solos, what does that mean?

Alot of artists use feedback in songs. Bands like Nirvana etc used it pretty regularly ie the end of Anueryism (I dont think thats spelled right). For a feedback solo try Drown by the Smashing Pumpkins thats got a big feedback solo at the end of it.
Basically feedback is 'white noise' which is picked up by your guitar played through the amp which in turn is picked up by the guitar even more and then played through the amp. This happens very quickly and you get a distinctive howling screech kinda noise.

Hope that helps

Neal