The purpose of a pool cartridge filter is to clean your pool water by capturing debris and decontaminates in the folds of the filter. With time these folds will become clogged, water will not flow thru the filter as easily, and pressure within the filter will increase. Operationally, with water flow down, you will not be turning you pool water as frequently and your pool decontaminates will start to build up. It's time to clean your filter. As a general rule of thumb, your filter should be clean at least once a month during the summer and every other month if your pool is operational during the winter. Although this description is specific to a Hayward cartridge filter, many of the steps can apply to other models of cartridge filters.
Next you will have to relieve the pressure that has built up in the pool filter. Note the pressure on the pressure gauge. It will typically read between 15 - 20 psi. To relieve pressure turn the air relief valve on top of the filter counter clockwise. Water will spray out initially as you see the pressure on the pressure gauge go to 0 psi.
When replacing the filter top, watch that the O-ring doesn't bulge out of its grove. This becomes more of a problem as the O-ring becomes older and stretches. If the O-ring is not set properly the top will not close correctly and water will spray out when the pump is restarted. The O-ring should be replaced if this is a recurring problem.