Plain English Meaning
Things a student pilot must get right before being allowed to do them alone, because getting them wrong could lead to an accident.
Full Definition
A category of training items considered critical to the safe conduct of a flight, requiring that the student demonstrate proficiency before being allowed to perform them solo or be signed off on them. In a training syllabus, safety-of-flight items are flagged for special emphasis because mishandling them could endanger the aircraft or its occupants.
Context Anchor
Seen in training syllabi, lesson plans, and instructor decisions when a planned must be adjusted to keep the flight safe.