What does this circuit symbol mean?

That signifies a jack with a connection that breaks when a plug is inserted. In the case of your schematic, that connection isn't connected to anything anyway, so it shouldn't matter.

Barrel connector with switch -- 2 conductors, 3 contacts.

Here are some similar parts Digi-Key has listed. Most will show a diagram in the data sheet.

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Terminal 1 is the sleeve connection, and terminal 2 is the tip. Terminal 4 is a switch contact that is connected to the tip when there is no plug inserted, and disconnected when the plug is inserted.