Vacuum tubing from manifold vacuum. Here is the schematic on my 2000 Expedition with the 5.4 liter.
On my 2000 the white plastic tube goes to the purge valve mounted on the driver side firewall as pictured here.
The red plastic tube runs to the fuel pressure regulator on the driver side fuel rail.
The third line runs to the EGR solenoid.
If your tubing melted into two pieces, cut off the damaged ends and rejoin them with some rubber tubing. Available at the parts store you will need around a 1/4"OD and 3/16"ID.