When we learn new information, we remember it best immediately after we have learned it. We then forget details as time passes. Even after a few days, we may be able to recall only a little of what we initially learned.
To remember what we've learned over the long-term, we need to move information from short-term memory (what we're currently thinking about or aware of) into long-term memory.
To do this, we need to review what we've learned, and we need to do this often. It takes time to commit information to long-term memory, and reviewing information helps us do this.