Technique
If you've never played the piano, perhaps you have watched someone play. They move their fingers and hands in a coordinated way to produce music. The way they move their hands and fingers is called technique.
A piano has 88 keys - 52 white and 36 black keys. The average person has 10 fingers. Students start by positioning their hands over 10 keys. Through learning the techniques behind playing piano, they learn how to re-position their hands while playing so they can play more than just the 10 notes where their fingers are positioned.
The piano also has foot pedals. Technique also covers the use of the pedals. (Most pianos have a sustain pedal. The student typically learns how and when to use this pedal after they have learned basic keyboard techniques.)
Summary
Technique will help you learn how to play. Theory will teach you about what sounds good to the ear. Put them both together and you have a formula for creating good sounding music.