What is the symbol on Ardunio Uno schematic?

it is this on the PCB.

a pads to solder the connection back if you needed to cut the trace before to disable the automatic reset on USB connection

Probably solder pad jumpers - you should be able to apply a "blob" of solder across pins 1 and 2 to produce a short circuit hence, the one you show might give you the option of adding an external capacitor to a reset pin.

But if you really want to be sure, open the EAGLE PCB file associated with this EAGLE schematic and look and see.

Those are 2 pads on the PCB that are close to each other.

On an official Uno board, there is a trace routed between these pads that you can cut to break the signal, and if you want to restore connectivity between the nodes, you just solder a blob onto those pads.

You need to cut it if you don't want to uno to restart when you attach the USB cable