Early exercise tax is the tax treatment of exercising stock options before they have vested. By default, the bargain element (FMV at vesting minus strike price) is ordinary income at each vesting event going forward (and AMT adjustment for ISOs), unless the holder files an 83(b) election with the IRS within 30 days of exercise to recognize income at the exercise date instead, when the bargain element is typically zero or small. It is the technical reason why early exercise without 83(b) is worse than not exercising, and why the 83(b) filing discipline is so important.
The tax mechanic of early exercise without 83(b):