IPA: /ˈkaʊn.sə.lər/
KK: /ˈkaʊnslər/
A person who gives advice or guidance, especially in a professional setting.
She decided to see a counsellor to help her with her anxiety.
Counsellor is formed from "counsel" (from Old French *conseil*, meaning advice or consultation) and the suffix "-or" (meaning a person who). A counsellor is therefore a person who provides advice or guidance.
Think of someone who gives 'advice' ('counsel') — the '-or' indicates that this person is the one providing that advice.