“Overboard” (por la borda) is a nautical word.
Sometimes mortals “fall overboard (caen al agua), that is, they “fall over” the side of the ship or boat into the sea. Perhaps a seaman would shout “man overboard” (¡hombre al agua!) so that a rescue should start.
There are also some idioms (modismos) that use the word. The context is something that is exaggerated or excessive.
• Let´s not go overboard (no hay que exagerar) about the negative effects of Brexit. If someone goes overboard he is exaggerating, in this case, the effects of Britain leaving the EU.
• You can have some wine but do not go overboard (…pero tampoco en excesivo), meaning that you should not drink a lot of wine.
• Mr. Sánchez went overboard (… tiró la casa por la ventana para …) in the arrangements for Mr. Podemos´s birthday party. This means Mr. Sanchez arranged too much and it was excessive.