A block is a dedicated period to class time at school. Every school day consists of four blocks that are 1.5 hours each.
In VA Beach, some classes, usually core and honors, last only one semester. However, you have them for 1 block or 1.5 hours every day.
Other classes, mostly AP and dual enrollment, last all year. However, you have them every other day again for 1 block or 1.5 hours.
Some high schools use both systems, so you may have semester and year-long classes simultaneously. Others only use one of the two systems, so all classes are either semester or year-long, and it doesn’t matter if they are core, honors, AP, or dual enrollment.