Wet dreams occur when dreaming about sex. You crack a boner and if the dream is good enough you blow your load straight into your pyjamas or bed sheets. You might wake up to the front of your boxer shorts feeling stiff. This is because the semen has dried during the night.
It is nothing to worry about. If you hate cleaning the sheets in the morning, try getting in a wank before you go to bed. This may or may not help depending on how horny you are. The most annoying part of wet dreams is you rarely get to remember them. Oh well.