Non-Shunted
Non Shunted or unshunted allows the wiring of one or two wires from each side of the socket as shown on the left side of the photo below.  Non-shunted typically has 4 holes although with some newer sockets it may only have two holes with one on each side. In this setup the pins on the lamp do no make contact in the lampholder.   Non shunted lampholers are necessary for most ballast bypass T8 LEDs.   The load and the neutral will be wired into one tombstone.  The lampholder on the other side is a dummy or placeholder.   This is referred to as single pole wiring.

Shunted
The shunted lampholder typically has two holes on one side as shown on the right side.  Inside of the lampholder, there is a copper shunt or jump which connects the two sides allowing the pins on the lamp to make contact.

Existing fluorescents use both so it's best to check if you need them.