5 times five equals 25 is the same as 5*5=25. This is math terminology, more specifically this is multiplication.
One at a time refers to doing/handling a plural number of things each individually. for example if you were a waiter working in a restaurant and you had a tray of 10 water glasses to serve to ten people sitting at a table. You wouldn't just place the tray on the table and tell people ' everybody grab a glass!' No, that would be rude. you would hold the tray in one hand and serve them to people "one at a time" with the other hand.
Let me know if this helps or if you need more examples!