No entrance!

Вход - an entrance. This word is easy to meet on a metro station door or a shop door. Be carefull of mixing it with выход - the exit.

Запрещен - is forbidden. You can see this word in some other combinations, as фотосъемка запрещена (No photo!).

Посторонний means a person, who doesn't live or work here, an extraneous person

So, if you don't feel, you have a strong connection with the place, which door has such a sign, don't enter))