A polyphonic synthesizer is more like an organ. A piano is a percussion instrument, hammers striking strings. An organ is a woodwind, wind blowing through pipes.

How hard and fast you press a piano key has an impact on how that hammer strikes the string. With an organ, it's a little bit different, more like opening a valve to let the wind through.

A monophonic synthesizer only sounds one note at a time, so if you press more than one, it changes from one note to another. How it handles those changes can be controllable, leading to some unique playing styles.

The keys look the same. The playing technique is different. That's the simplified version. However, there is a lot of variation between synths. This answer is not true in all cases.