In my experience, "summer hours" mean half-day Fridays over the summer. I'd never heard the term until I moved to NYC.

To elaborate: Apparently the idea is that people could use it as travel time for weekend trips. Drive up to the Hamptons on Friday after lunch, have a full Saturday/Sunday to relax at the beach. Though I use it primarily for afternoon naps.

Summer hours could also mean that the expectation that employees have the option of taking the entire Friday off during the summer, provided that you schedule into your work Mon-Thurs. to make for the lost "eight hours unit" across those four days of work (e.g., WFH, come in an hour earlier).

Summer hour to me always meant it wasn't uncouth to leave an hour early during the week or take a full day or half day on Friday. Really it's just more relaxed hours.

37 signals has a 4 day work week during the summer months.