Interest rate
The rate central banks charge commercial banks. Higher = stronger currency (usually).
Core forex fundamental. Higher yields attract capital (long term) and command risk premium. Rate differentials drive carry trades. Every major data release ultimately gets priced as 'does this raise or lower rate expectations?'.
Learn to actually use Interest rate.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.