If you have a MBPro 2015 model, then most likely the Trackpad Cable is bad and needs to be replaced. This would be the first easiest step and least expensive option. The cable on the 2015 has a high rate of failure after several years of use.
If you have a different model, then the Trackpad itself may be bad.
You need to have Apple or an Apple Authorized Service Provider examine the laptop and provide you with an estimate for repair.
Sometimes rebooting the laptop or performing a PRAM Reset will temporarily fix the problem. Hold the PRAM Reset for at least three chimes or a full minute if you have a newer laptop that doesn't chime.