What is required is a way of keeping the turbine of a turbo operating at a constant speed so that the compressor efficiency can be maximised. R emember that turbo turbines like to run at a single speed. However, as measuring turbo RPM is not practical, and as boost level is directly related to turbo speed, keeping the boost constant is the purpose of a wastegate.

The wastegate is just a valve that opens when the required boost level is exceeded, and allows exhaust to flow around the turbine, instead of through it. This lowers the pressure differential across the turbine, less work is done, and the turbo slows down.
The only issue with a wastegate is that it must be able to flow enough gas to let the turbo slow down. If it can't, then you get "boost creep" where boost levels slowly grow as the car remains under boost , which is not good.