It means that the preamp - the part of the amplification process that produces and shapes the tone of your electric guitar - uses low-power "preamp tubes" to do so. In most guitars it's an 12A_7 or an ECC8_.
Hi, I'm Peter
No. That's just a portion of the amp. I'm guessing you're asking about the Valvetronix series, which are NOT tube amps- they're hybrid. Which is to say they sound pretty much like solid state, not tube amps.
There are all tube preamps (Mesa Boogie Quad), but it doesn't make up an entire tube amp. You also need the power amp section and a cab. If you're using a tube preamp, you'd want to pair it up with a tube power amp, that'll really produce the 'balls' in your sound.