digress

(v) lose clarity or turn aside especially from the main subject of attention or course of argument in writing, thinking, or speaking
- She always digresses when telling a story
- her mind wanders
- Don't digress when you give a lecture
divagate, stray, wander,
(v) wander from a direct or straight course sidetrack, straggle, depart,

divagate stray wander sidetrack straggle depart

digression digressive