What exactly is "technique"?

Keeping pad level low. Using hands rather than your chest when fighting blocks, coming off the ball low, using swims and rips, using your eyes and keeping your head in the correct gap. Stuff like that.

Basically most HS elite D linemen are either exponentially faster or exponentially stronger than their opponents so they don't use any moves (swims, rips, etc) because they can just beat people with their athleticism. In college they have to learn these things because everyone is good in college.

Keeping your body in the optimum position to make the play.
Bad Technique:
Quartergbacks throwing off their back foot.
Running backs running upright.
Linemen getting their pads too high.