The tron legacy code was the uncompiled code for the updated Tron program. It was written to clear the grid of any unauthorized users. Recovered by Jet and compiled into Ma3a, the legacy code caused Ma3a to grow to immense size and attack all nearby users.
It was also present in Tron: Ghost in the Machine. Mercury claimed it was present in Jet's disc and he needed to use it to defeat the MCP. It was later revealed as a trick, as it was all in Jet's head.