An interim is a period of time between one event and another. If a teacher leaves mid-year, there might be an interim teacher, one who takes over the class until a permanent replacement is hired.
Interim is a Latin adverb meaning "in the meantime." The first part, inter means "between." Interim is the time between, and you can use it as a fancy way of referring to a time you squeeze something in. Maybe you get ice cream during the interim between school and piano lessons. This word is often used in official contexts — for example, after the CEO of a company steps down, an interim CEO might serve while a formal job search is conducted.