Learning how to read schematics, questions about fractional voltage in schematic
Look at the top of R20 - that is labeled V and is the supply rail. (V is also connected to 9VDC which is the power input - see the DC connector and battery, towards the top left of the schematic.)

Therefore, as commented by jonk, the node at the junction of equal resistors R20 and R21 must be half of V hence 1/2V means exactly that.

Also, looking carefully, the arrow symbols labeled 1/2V are slightly smaller than the arrows which are the ground symbol. On the full schematic you can compare their size and see the difference - but otherwise, that choice of arrow by the designer could easily be confusing! As kindly pointed out by Kevin in the comments, that smaller arrow is being used here as the symbol for a named node.