Welcome to healthcare magic
From your description it appears to be a simple boil (Furuncle). Furuncles
are usually painful. You forgot to mention whether it is painful or itchy. If it is small topical application of antibiotic cream like mupirocin
2% thrice a day will suffice. Shaving or waxing under arm hair might be a trigger for new boils
It is not anything dangerous.
Hope this helped