Retakes. Say a line badly, stop, and say it again. Studio keeps the take you meant, which is almost always the last one, and drops the rest.
Run-ups and false starts. The half sentence before you found the thought.
Filler. The ums, and the production chatter that is obviously not part of the video.
Tangents. Whole sentences that match nothing in your script, when you asked for a cut that follows it.
Dead air. The pause where you were reading the next line, trimmed to the pace you pick.