120 reset means to unplug a computer in order to reset or unjam it. The 120 comes from the fact that 120 volts is the wall socket voltage in the US. 120 reset is pronounced /wuhn-twen'tee ree'set/.
... but not in the United States.
However, because the GBA has no region lockout, European games will work fine on a U.S. GBA unit, and apparently, even a player who knows no ...