A tech is something you put into a deck to either:
help with a bad matchup, or
counter something that's prevalent in your metagame.
For an example of 1, putting an Electric Pokemon into an otherwise all-Fire deck to help deal with matchups against Water decks. Or adding a Dugtrio CG to a Honchkrow deck to prevent weak Murkrows on the Bench from being sniped.
For an example of 2, see Yamato's Fast Empoleon deck for Worlds last year. He included an Infernape because Japan was rampant with Scizor ex, which is a OHKO for Infernape.
Thank you, that's about what I thought, but it's nice to have it confirmed.
I'd bake you some cookies, but due to, most likely, far living distances, they'd be stale by the time you get them... So I'll give you my thanks... And this: