A tech is a card that isn't part of your main strategy, but usually helps you against bad matchups vs. another deck. For example, many decks tech in 1 or 2 Crystal Shard to take down the powerful, weak-to-Colorless dragons like Salamence and Salamence ex.
T1/T2 means turn one, turn two, repsectively. T# means "turn #". A phrase like "T2 100" would mean doing 100 damage by turn 2.
The metagame is something like what decks are currently popular and used at the moment. It can also be used as a verb; metagaming is tweaking your deck based on the decks that are popular in your area. It has its risks, however; if you guess wrong, you'll probably be screwed.
We don't know how they will handle things when D/P cards come out. Only thing we can do is wait and see.