Being relatively new to the world of JA, I was wondering what exactly was meant by the passage I put below in quotes in P&P in regards to what it meant to buy a commission and what a commission actually is. I assume it has something to do with the military but am not 100% sure of this. Plus, how does one buy it and how much would it probably cost?

"His debts are to be paid, amounting, I believe, to considerably more than a thousand pounds, another thousand in addition to her own settled upon her, and his commission purchased."

Hopefully this is the right place for this question and thank you in advance for any information.