It's the motto of a good Scout.  There, it means to be prepared for anything.  If you have your trusty Boy Scout knife on you at all times, then you are prepared to open cans, punch leather, remove screws, and get arrested for trying to take a weapon on an airplane. 

As long as you are speaking in specific terms, saying to be prepared is a good thing.  We can be prepared for bad weather, such as putting the storm shutters on or being out of town.  We can be prepared for natural disasters by having our seven days of food and water in reserve.  We can be prepared for automotive problems and household fires and having snacks ready for our ravenous children when they get home in the afternoon. 

It's those intangible things that being "prepared for" are just... well... silly.  Take, for example, the phrase, "Prepare to be impressed."  How do you do that?  Do we need to stretch out first?  Make sure that nobody is standing too closely so when we jump in the air and throw our arms out we don't smack them?  How do you prepare to laugh?  To cry (other than having tissues close by)?  How do you even prepare to die?  Sure, you can make peace with your god, if your religion allows that sort of thing.  You can have your will drawn up and pre-arrangement plans secured.  But when it gets down to it, I'm not sure anybody is truly "prepared" to die.

If you're going to tell somebody to be prepared, then, make sure it's something they can actually prepare to do... or don't tell them to be prepared.  Yeah, if your friend is contemplating setting his clothes on fire and then jumping off the top of a ten story building with nothing but an umbrella... 'cause, you know, the burning clothing will create an updraft of warm air that will slow down the fall of the umbrella, and he'll be on the ground before the fire really starts to burn him, and then he can just roll around on the ground to put it out...  it probably wouldn't hurt to tell him he's probably going to die.  How he might want to prepare for that is totally up to him.