I’m reading The White Company by Arthur Conan Doyle, and it tells of the English going to fight in France and Spain in 1366. The main character is not a noble, but he was made a squire to a famous knight. Their little army has 80 archers and 20 men-at-arms. I doubt that all these men-at-arms are nobles, but would they have squired for someone at some point? How did they gain the money to buy plate armour and learn how to fight if not raised in the way the nobility were?
As one of our linked answers below explains, a knight is a man-at-arms, but a man-at-arms is not necessarily a knight. If anyone would like to expand further, please don't let this post stop you, as more posts are always welcome! For the meantime, OP, here's two posts that explain the term...insofar as anything can have a fixed meaning in the Middle Ages.