Being the first to raise when no one has entered the pot.
"Open" means the same thing, and the share of hands you can play differs by seat, with later seats playing wider. Example. From UTG you open AJo or 66 and fold 72o.
At the table
If everyone folds from UTG and the CO raises to 2.5bb, the CO has opened (RFI). A raise after someone has limped in is not called an open.