Pinning an IP lets you choose an IP that you want to connect to every time you connect to that particular location, it's handy if you find an IP that is consistently fast for you or works well for you for latency purposes, it lets you have a shared static IP.
STEP 1) Connect the app to your prefered location.
STEP 2) Once connected you will see a little "pin" icon on the right side of the connected IP - click it to Pin the IP.
STEP 3) The "Remote IP" text will change to "Pinned IP" and the little icon becomes lighter as in the image below.
If you hit disconnect you will see that every time you switch to this location, in our case Dallas, you will connect to the IP you have pinned as in the image below.
To remove the pinned IP just click the little cross icon on the left side of the pinned IP and you will then be able to connect to dynamic IP's again.