Behold, I am going to come like a thief! Blessed (happy, to be envied) is he who stays awake (alert) and who guards his clothes, so that he may not be naked and [have the shame of being] seen exposed.
What I believe God is saying here is:
Nobody on earth knows when God will return, so be on your guard, do not sleep, stay awake so you do not fall into sexual impurity. Guard your clothes. DO NOT let anyone take them off you so that Christ returns and finds you exposed and in sin.
Stay awake and alert. Be blessed.
Soli Deo Gloria.