This slang phrase simply means that you shouldn't drop your soap whilst in the shower because it might break, and nobody likes small soapy chunks in their shower.
I dropped my soap and I had soapy chunks in my shower for months.
Dont drop the soap!
prison slang for: dont be someones bitch. you will get bum raped if you "drop the soap"
e.g.(your bending over in the shower) dont drop the soap richard