It means "good at", followed by a noun, pronoun or verb ing. Be varies with person and tense.
He is good at English.
He is good at English.
The difference between being good at and being good at usage
Good at and good at, these two phrases have similar meanings, and good both means "good at". The difference lies in the prepositions at and with. The former emphasizes "being good at ……", and its object often refers to words such as "skills, skills, disciplines, etc."; The latter emphasizes "being good at getting along and dealing with ... for example:
Most students in this class are good at math.
Most students in this class are good at math.
She is always kind to unfortunate people.
She is always friendly to unfortunate people.