what does it mean ?
According to the OP's comment, the pump has a 7 bladed impeller. Each blade will cause some disturbance to the pressure and/or velocity in the flow. Those disturbances will cause a force on the mounting of the pump.
The text on the OP's picture says the pump RPM was 2940, or approximately 50 revolutions per second. The frequency of the disturbances in the flow is therefore about 350 Hz. Maybe the pump and the structure it is mounted on has a natural vibration frequency close to 350 Hz moving in the vertical direction, and that is why you are getting a large amplitude on your plot.