The "Monthly Search Volume" column lists the average number of monthly searches for that term, according to Grepwords. In my testing, this is very similar to the volumes that Google's keyword planner would give you.
The "Context" column link is a simple helper link that opens up a Google search of that keyword (and your chosen subreddit). This will help you when it's not clear how the keyword might be used.
There are a few main ways to use the keyword results. First, you'll hopefully find some low competition keywords to target directly. Additionally, you could feed this list of keywords through other keyword research tools to get an even bigger list of keywords that you might not have found originally. A final idea is to use the results to create accurate audience personas.
If the subreddit you're looking for doesn't come up in the auto-suggest list, it's too small (<10,000 subscribers). Subreddits with lots of comments tend to have more comments than less active ones. If you think there's been an error or really need more keywords, contact me and I'll see what I can do.
Keyword volumes come from Grepwords, which is one of the best data sources at the moment as far as I've seen. You're free to export the keywords and run them through a keyword volume tool of your choice.
I'm primarily a freelance writer and web developer, but I like to make small tools around my interests when possible.