What is meant by United we stand Divided we fall?

Basically that there is safety in numbers; when a number of people gather together to face a problem or situation, they are more likely to succeed than a few people who stand alone.

This can be used in a number of contexts, for example:

• People of a neighborhood gathering together to fight crime in the area.
• Thieves giving the same alibi to cover each other.
• Acts of war where allies gather against a common enemy.
• Pioneers crossing an uncharted area, where together they stand more chance of survival.