Answer :

When Julius Caesar crossed the Rubicon, it was an act of treason towards Rome sense the senate warned him beforehand to disband his army and then cross the river. So once he crossed it, It was a blatant act of defiance towards the senate. Caesar himself knew that there was know turning back, once he crossed it, he was immediately deemed a traitor, and if caught, punishment would be either crucifixion or being burned at stake.  
it was important because when he and his army crossed the Rubicon river and entered Italy it caused the civil war.