Extended Techniques are (to oversimplify) ways of playing your instrument that are unconventional.
For flutists they tend to be multiphonics (playing two notes at the same time or playing + singing two notes at the same time), harmonics (overblowing a traditional fingering to get higher pitches that have a different sound than their traditional fingerings create), key slaps that create a percussion effect, beat-boxing, flutter tongue, whistle tones, etc.... Many more extended techniques exist, but usually flutists will start with one of these.
Aside from the new sound opportunities, extended techniques give flutists exciting ways to develop embouchure flexibility and breath control.
If you are trying to learn extended techniques without a teacher, it can be more difficult to begin. Here are some resources you may find useful when you get started:
Ellen Johnson Mosley is a flutist & fan of fine television.