Jacked In = connected to my hacker rig
Run = attempt to hack and steal hidden data
Megacorps = every evil international corporation ever
ICE = Intrusion Counter Electronics (in other words, defensive software such as firewalls, aggro tech, anti-virus software, trace software etc)
Rezzed = installed and resolved
Flatlined = braindead, or a successful trace lead to you taking too much meat damage and being killed the old fashioned way
----------
Image: Shibuya Night, Wikimedia Commons
----------
For a totally different take on a hacker attempt listen to:
https://soundcloud.com/dacca/netrunner