let (someone's) mind wander

Translations
позволить мыслям блуждать
During the lecture, he let his mind wander and missed important points.
Во время лекции он позволил мыслям блуждать и пропустил важные моменты.
Definitions
To allow one's thoughts to drift away from the current task or situation, often leading to daydreaming or distraction.