To pin a portal is to deploy resonators on a portal. This is down to reduce time typing in chats when sending instructions.

Example:

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