They mean that mIRC can currently be run off a usb device, such as a flashdrive and so on.
As for U3, a quick search came up with "New technology from a company called U3 allows a drive to store and, when plugged into any PC, securely run applications--without leaving a trace of data on the host computer. The applications (which must be U3-compliant), data, and personal settings all reside on the portable drive, permitting you to temporarily turn any Windows 2000 or XP system into a personal workstation without threat to your privacy."
I believe the suggestion is a U3 compliant version of mirc, or for mirc to be made u3 compliant.
