It typically means that they are activating their subscription feature because they're automatically set up to subscribe to each thread to which they post. Subscribing to a thread means they'll be notified (either via email or here on the site) when there are new posts posted to the thread in question.
Here is our subscription tutorial for a lot more information.
One way to see your default subscription setting is to click on My Profile and then find the Edit Account Details button. You'll then see:
That said, you don't actually have to actually post to a thread to subscribe to it! Members are also able to click the Subscribe button right under a thread's title to make subscription decisions.
Hope that helps explain what people mean by saying "subbing." It does seem odd until you know what they're talking about!
As you browse the site, you'll have the chance to subscribe to forum threads, users, wiki articles, and other things that you find interesting. By setting this frequency, you specify how often you'd like to receive these email updates. You do have the option to change this frequency for individual subscriptions on the edit subscriptions page.
You can opt to receive email updates:
- Immediately: A brief email describing changes will be sent to you within seconds of the change occuring on the site -- note that this may generate many emails per day if you subscribe to frequently updated content.
- Daily Digest: You will receive a single email up to once per day detailing any updates to your subscriptions in the last 24 hours.
- Weekly Digest: You will receive a single email up to once per week detailing any updates to your subscriptions in the last 7 days.
- Site Only (No email): You will see updates to subscribed items in the activity feed on your personalized homepage, but you will receive no subscription email.
- Don't Subscribe: Choose this option if you do not wish to be subscribed to items by default.
- Automatic: Choose this option if you would like the site to choose the most appropriate frequency, based on the kind of item you're subscribing to.