Both terms describe the ability of a spindle not to "slip" on its own when at a standstill.
Static self-locking refers to a thread angle between 2.4°-4.5°. In standstill, the spindle does not slip on its own, but it might not come to a stop on its own from motion. Dynamic self-locking occurs when there is an additional braking action from the dynamics. The thread angle is then smaller than 2.4°.