So I'm very new to vex and apologise in what is probably a very simple question but what is main purpose of geohandle?

geohandle() is used as argument for vex functions to indicate which wrangle node you wish to apply that function too; And in such cases it always means the wrangle node that contains that function with geohandle specified.

In many functions geohandle() can replace using 0 as an argument to indicate which input of the wrangle node to use.

Thankyou, very well explained