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.
The impeller blades will always make a vibration. Unless you see an increase in the amplitude of this peak in your spectrum, you may just be seeing normal operational vibration. Monitor it for increases. If you see increases in vane pass frequency that are significant, you may be looking at a worn impeller or a possible clearance issue between the impeller and the housing. The short answer is that if you don't see an increase in this, I wouldn't worry about it.
You do need to ensure that that peak is actually seven times running speed and not a non-integer multiple of running speed. Non-integer multiples of running speed could indicate a problem in a bearing. Look to see if your waveform is sinusoidal. If you see many impacts or discontinuities, you may have metal on metal impacting. If it is a sinusoidal waveform it is more likely to be turning speed related, such as the vane pass frequency.