There are a variety of ways to tell LAN Speed Test where you want to test and this is the place to do that.
If testing without LST Server, then you enter the location of the shared folder to test to. The easiest way to do this is by clicking on the '...' button and finding the shared folder from there. You must have read/write access to this shared folder. You can also enter the info manually or select a recently entered folder from the drop down box.
Here are some Windows examples...
-> N: (Root folder of your mapped drive)
-> N:\Files (Shared folder 'Files' of your mapped drive)
-> U: (Root folder of your USB drive to check the speed of that drive)
-> \\Server\Files (Shared folder 'Files' on computer named 'Server')
-> \\192.168.0.1\Files (Shared folder 'Files' on computer at IP address 192.168.0.1)
Here are some Mac examples...
-> /Volumes/Files (Shared folder 'Files')
-> /Volumes/Public (Shared folder 'Public')
-> /Volumes/Samsung USB/LAN Speed Test (USB Flash Drive)