I've written about impressment several times before. These links below might be of interest, but to briefly answer your question, impressment was the practice of conscripting men to serve at sea. It took various forms throughout the 17th-19th century, and various groups of men were at times targets of or exempt from impressment. But roughly, the idea was that all men who were seamen or worked at sea were eligible to serve in the Navy and obliged to do so at moments of national emergency.

Take a look at these links and see if they help:

https://amp.reddit.com/r/AskHistorians/comments/53zfbm/are_there_examples_of_how_the_british_royal_navy/

https://www.reddit.com/r/AskHistorians/comments/2oo91z/during_the_age_of_sail_how_reliable_were/

http://www.reddit.com/r/AskHistorians/comments/2xwhrj/how_did_press_gangs_determine_eligibility_for/

https://www.reddit.com/r/AskHistorians/comments/3pn7v5/what_changes_occurred_in_the_construction/

Edit: also, looks like maybe autocorrect got you. The late Patrick O'Brian, born Richard Patrick Russ, is the author; Patrick Bamford is an English footballer.