A period is one session in a day. For example, a timetable for one school day may look like this;
1st Period - Maths
2nd Period - Science
3rd Period - Social studies
Lunch
4th Period - Music
@redlightakari
I agree, not all high school subjects are useful for everyone, but before completely giving up on every class, decide on what Job you want to do after school. Then, make sure it is achievable and and ensure you have the skills to get a job in that field. It really sucks to see students completely give up at school and then be stuck with a miserable future because they have to work a job they absolutely despise. Remember that to work in Japan you need a University degree to get a visa.