It's a shortened way of saying "24 hours a day, 7 days a week".
Here are some examples of how it's used:
* "The new coffee shop is open 24/7." This means the shop is open all day and all night, every day of the week.
* "I'm feeling stressed 24/7 lately." This means the person is feeling stressed all the time.
The phrase is common in both spoken and written English, and is often used to emphasize the continuous nature of something.