Be Prepared is the motto which was given to The Guide Association and The Scout Association by their founder, Lord Robert Baden-Powell at the beginning of the two movements.
This particular phrase was chosen by Baden-Powell as he found life kept giving him a constant flow of suprises which needed to be dealt with. He saw many people become defeated by the challenges which life threw at them, however the ones who managed to triumph over difficulties and turn near disaster into a positive experience were the people who were prepared beforehand.
Because of this, Baden-Powell chose Be Prepared as a motto for the Guides and Scouts, and a motto for life.
Being someone who is prepared can involve quite a lot of work and learning. It not only means that you should be able to deal with emergencies and prevent accidents, you should be equipped enough to deal with problems which you may encounter, or improvise to find a helpful solution. Thinking promptly and acting on a well thought out solution is also a handy skill to have.
To be able to put these skills into practise, you also have to be reliable enough for people to be able to depend on you if there ever is an emergency and able to stay calm whilst still being brave. Above all other things though, you must be able to put others first, whilst not putting yourself in jeopardy.
From my own knowledge
The Guide Handbook 1973 edition