It implies that someone is ready to fight, either physically or metaphorically.
Here are some examples:
* "The citizens took up arms to defend their city from invaders." (This means they physically picked up weapons to fight.)
* "The environmental group is taking up arms against the polluting factory." (This means they are engaging in a fight, but likely not with weapons, but with legal action or protests.)
The expression is often used in a context where someone is fighting for a cause they believe in. It implies a sense of determination and urgency.