New to this - what is keyword difficulty in seo?

It's a score, usually 0-100, that tools like ahrefs or semrush put on a keyword to guess how hard page one will be. It's mostly based on how many sites link to the pages ranking right now. Each tool works it out differently, so a 30 in one isn't a 30 in another. Treat it as a rough filter, then actually look at the results. If forum threads and thin pages are ranking, it's easier than the number says.