In terms of my comics a "buffer" refers to all the comic strips I've completed that have yet to be published online. It's all the comic strips I have sitting on my hard drive waiting to see the light of day.
When I first launched Doctor Necromantic I knew I wanted to maintain a buffer of strips just in case. In case of what? Well, whatever might come up. There are plenty of webcomic creators who are posting their strips as they get created. The problem with that is anything that might happen in one's life could easily disrupt the publishing schedule. I figure that if I want to maintain a schedule of updates I need a little wiggle room. Getting sick or injured, unplanned visitors, trips, just not feeling creative the day I sit down to work can all lead to delays in a strip getting finished.
Before I uploaded even the first strip I created about 5 weeks worth of comics. So far, I've been able to maintain that buffer, give or take, since launching in January. I currently have 6 weeks and a day in the buffer. Mentally it helps keep me on track and on schedule to chase that buffer more than the publishing schedule I've set out for myself, uploading a new strip 3 times a week.
On top of the buffer of completed comic strips I maintain another buffer of scripts for the comic. My process includes sitting down to write out scripts for the comic ahead of time and I often will give myself what I call a "writing day" during which I will just write out as many scripts for comics as I can. This helps me course out the direction for the comic and also gives me a little distance between the initial idea and drawing it out. That "second look" at the script, when I look at it to draw it out, gives me a chance to edit the dialogue, to try and tighten it up, make it clearer or funnier if I can.
Just having this creative process in place is helping me to keep on track with the buffer and, so far, the actual comic update schedule.