What they knew
Caesar's command in Gaul was ending. The Senate, led by his rivals and backed by Pompey, ordered him to disband his army. Without it he would face prosecution and likely the end of his career.
Roman law forbade a general from bringing an army into Italy. The small river Rubicon marked that line.
The choice
Caesar had only one legion with him, the Thirteenth. Crossing meant civil war. Staying meant submitting to enemies who wanted him ruined. Ancient writers report that he hesitated at the bank and then quoted a line about dice being thrown.
The consequences
Pompey and much of the Senate fled Italy. Caesar won the civil war, was named dictator, and was assassinated in 44 BC. The Republic never recovered. "Crossing the Rubicon" still means passing a point of no return.