Essentially beer money is sites that earn you small amounts of money by doing surveys etc.
I started using Prolific.ac a few months ago and have made £100, and haven’t invested much time into it. Just open it in my free time and do any surveys that come up. Pays for my lunch and doesn’t require much time.
Any other suggestions? I use Mturk but they only pay on amazon.com gift vouchers, but it’s useful if I want to order something from there.