This is the 65th most frequent Hungarian word.
jól
"Jól" in English means "well."
In this sentence, 'jól' is used as an adverb to describe the manner in which 'érzi magát' (feels).
Nagyon jól érzi magát a partin.
She feels very well at the party.
Here, 'jól' modifies 'számoltam' (counted), describing the correctness or accuracy of the action.
Ha jól számoltam, tíz ember van itt.
If I counted correctly, there are ten people here.
In this sentence, 'jól' is used as an adverb to describe the appearance conveyed by 'néz ki' (looks).