Tables in a database are used to store data, so the qa_posts table stores all the posts in the site, qa_users stores all the user accounts, and so on. There are about 30 tables in Q2A. Plugins may add more but it's very unlikely you'll get to 150.
It's also unlikely you'll reach 1GB of data unless you have a HUGE amount of content. The db on my site that's been running for years is still less than that.