It’s the mask opening in the F.Mask layer. It depends on the board manufacturer if they want you to set a larger mask opening than the pad, or if they want the opening to be identical to the pad and they adjust it themselves for their manufacturing tolerances.
When the mask layer is active in the Pad Properties, the value for this “Solder mask clearance”, i.e. the amount by which the mask opening is larger than the pad, can be set globally in Board Setup -> Solder Mask/Paste, for each footprint in Properties, and for each pad in Properties.
You don’t have to worry about that yet, only before you generate the gerber files to be sent to the manufacturer. If you want to know more about the mask layer, read How does solder mask layer work?.