US preschools often have several centers or stations devoted to different activities, for example a literacy center, a number center, a dress-up center, a science center, etc. http://fun-a-day.com/centers-a-basic-introduction/

yes I have heard it called circle time, which is the same description as Leigh said

thank you all for your answers!

Perhaps it means the time when all the children gather in a ring to sing and etc.