Club activities (部活動) are extracurricular groups at your school, that can range from sports clubs such as baseball, volleyball, and table-tennis to cultural clubs like tea ceremony, shodo (Japanese calligraphy) and more. Some schools may also have an English Speaking Society (ESS) Club, essentially 'English club'.
If your school does have an ESS Club you will most likely be asked to participate in running the club and providing content. While this may be extra work, students who participate in English club are normally extremely interested in English, and/or are driven to improve their English ability for their long-term future goals. It's also a great opportunity to interact with some of your students and get to know them better.
Interacting in school clubs is a great way to get to know your students and co-workers better. All clubs will have at least one teacher who advises the students. However it's important to know that some school clubs are highly competitive and may even compete on the National level. It is unlikely that you will be able to participate in a competitive club. Also note that club activities can be taken quite seriously, and if you join a school club you may be expected to attend regular, weekly practices.
It is also worthwhile to note, that while Middle Schools and High Schools will have school clubs, Elementary Schools will not typically have clubs.