Why did Jack give an irrelevant reply to Tess's question?

"Bootlegs" are recordings (in violation of certain publishing and copyright laws) made at live concerts by fans, which get circulated among the fan base in an "underground" fashion.

Think about people who go to movies new to the theaters, and record the movies on a video camera. Same kind of idea.

It gives the hard-core fans a chance to hear their favorite band in a live setting, unedited, and there's a bit of a reputation enhancement or cache to having something much less common than a studio-released album.

So, when she asks if he likes the band, the Vines, and he responds that he has bootleg recordings of their shows, it's an indication that he's a hard-core fan.

Merriam-Webster Dictionary: Bootleg