what's the mening of """at the side of""" ?

If the front of the house faces north, there is a primary entrance on that side. There may also be doors to the house on the east or west. These are side doors, or doors at the side of the house.
A door on the south would be a back door, or back entrance, or door at the back of the house.

Usually the front entrance is for guests and is more formal, and the side or back doors are more casual. When I come home carrying groceries, I enter through the side door because it's closer to the kitchen. In a fancy house, the side doors might be for service people like maids or caterers who are not guests but workers in the house.