Dude, I must tell you that there's no exact English explation of Jiayou. Usually, it means 'Come on!', 'Go, go, go!', or 'Hop on'. However, Chinese people could use it in different situations, such as, they shout out Jiayou before Tian'anmen to comfort and encourage victims of the Sichuan earthquake, and they shout it everywhere in the venues of the Olympics. Generally, it means let's put on some elbow grease，or go for it，or put more effort into it，or Don't give up，or C'mon, mate, you can do it, give it your best shot, etc.