If you take a look at your home you will notice it is enclosed by outer walls. All cells are enclosed within something called a plasma membrane. The plasma membrane is not exactly the same thing as the wall in your house, but it does hold parts of a cell inside. These parts of the cell are what biologists call "organelles." That is a Latin name for little organs.
Sometimes people think of cells as a balloon filled with fluid. That is not really true because a balloon does not let things move in and out like the membrane of a cell. It is important for cells to be able to move materials in and out of the cell.