What is phone line 1 and phone line 2?

Phone lines using the RJ11 jack (the phone jack) have 4 wires...Colors of those are black yellow red and green. The Red(tx) and Green(rx) are for LINE1 (which is the primary phone number, usually) The Yellow(Tx) and Black(Rx) lines are for LINE2 (Which is for the secondary phone line, which most Phone services do not include in their standarized packages).