"Lei" is the formal form of addressing someone, equivalent to "you" (singular) in English, typically used in professional or polite contexts.
When addressing someone formally with "lei," you conjugate verbs in the third person singular, just as you would with "lui" (he) or "lei" (she).
For example:
Lei parla italiano (You - formal - speak Italian)
Lei lavora qui (You - formal - work here)