advanced learner; experienced person
Refers to someone with a high level of skill or knowledge in a particular field, often contrasted with beginners (初心者) or intermediate learners (中級者).
This course is for advanced learners.
He is an advanced practitioner of programming.
上級者 refers to an advanced, skilled person, the direct opposite of a ビギナー who is just starting out.
intermediate-advanced learner; upper-intermediate student
Compound of 上級 (じょうきゅう, 'advanced level') and 者 (しゃ, 'person').