Hi darkballa,
Rectifiers, as others have said, are used to convert AC into DC. These thing are used in power circuitry in the amp, which is actually not in the signal path. Signal path is the path inside the amp where your guitar signal goes through from the input jacks up to the speaker.
Old amps (like Vox AC30 or Marshall Bluesbreaker) used tube rectifiers, because solid state technology was not popular yet. The problem with tube rectifiers is that they're very inefficient. So, the harder you pick a note, your sound gets more compressed. This, to players, creates a 'feel', as if the amp is responding to their playing.
Solid state rectifiers, on the other hand, is very efficient. So much that the 'sag' phenomenon the players usually get from their tube rectifiers equipped amp, is gone. Some people like this new tight sound, others do not.
Mesa's Rectifier line of amps, offer both tube and solid state rectifiers which you can choose by flicking a switch. However, these amps are not known for their vintage 'sagging' sound, but rather by their massive distortion which is produced by the cascading gain pre-amp, which ironically has nothing to do with the rectifier. Why Randall Smith chose the word 'rectifier' as his amp's name is beyond me.
The term 'rectifier technology' is simply strange; to me at least. :lol: