Evenings of the workday

Hi,

I was speaking to employees at the firm I'm joining to ask about the usual workday, and was told that people are generally free to do things like go for dinner, to the gym etc from the evening onwards but usually only officially leave the office at night.

Can someone tell me what that means? Is it common for people to go to the gym in the middle of the workday, and is that even a feasible thing to be doing? Just trying to figure out how the hours of the day are actually going to be occupied.

Thanks in advance, and appreciate any insight.