You can learn much by reading the historical posts, but a quick summary
dropped names are names that have been deleted as they have not been renewed by their current owner. This is either because they've finished with the domain or that they have ignored / not recieved the renewal reminders.
drop lists are simply lists of names due to expire and be deleted, many Acorn members (including us
offer drop lists for a fee so you can be aware of the names that will drop on a specified date and then attempt to catch the domain of your choice.
drop catch (or simply catch) refers to the process of detecting when a name is deleted and then submitting a request to register the domain. A name worth having is usually caught within a few seconds of dropping (often much faster if it's a valuable domain). You can either write your own software to catch domains or use the services of many of the members here to catch for you.
"free to reg" usually means that the poster had these in their lists but decided not to register them and rather than just discarding the list, they've posted the list for the benefit of others. The custom is that if you find this of benefit, you make a post to indicate that you've claimed the domain and then make a donation either to the person who posted or to the site running funds (or Admin's beer fund, not entirely sure
- There's a donation button on the front page.
To register, just go to your favourite registrar and register as you would any new domain.
Best of luck on your exploration of the world of dropped domains.