To pin a portal is to deploy resonators on a portal. This is down to reduce time typing in chats when sending instructions.
Me: can you pin that portal later
Noob: what do you mean pin that portal
Me: can you deploy resonators on that portal so the level of that portal gets higher
Noob: Oh, I didn't understand what you meant
Me: Pin = Deploy, i've spent much time spelling out the word deploy
Sorry do not understand this description.
I was not real sure what it meant either. It is not a term used in my local area. I've had a go editing it but if I'm wrong feel free to correct it.