As Caesar was the pro-consul in the province of Cisalpine Gaul he had legal authority to command his troops in that province, it was this, and his pro-consulship in Transalpine Gaul that allowed him to fight his Gallic War. While he could command the army in these provinces, it was illegal to take his army into another province. The Rubicon was the border between Cisalpine Gaul and Italia, so by crossing the Rubicon with his army he broke the law and committed treason, which sparked the war. Hope that's helpful.

Well at this point in time there was a pretty big confrontation that was building between Caesar and particularly obstinate members of the Senate. Basically Caesar had done things of questionable legality while he was Consul but could not be pursued in justice while he held a position of power that granted him imperium. At the Rubicon Caesar could either entire rome peacefully and lay down his post, thus facing prosecution that could end his career, or enter Roman territory with an army in a further breach of his formal powers.

He felt that the peaceful option would lead to his demise and believed entering under arms was the option forced upon him by an uncompromising senate.