ETTR is shorthand for Expose To The Right.
This refers to a common practice by pros of shooting raw images with exposure pushed high enough so that the histogram (preferably derived from a UniWB shot and examined on all color channels) extends to but does not clip at the right edge. The goal is to get the brightest highlight data just under the saturation point of the sensor, then use post processing techniques to change the linearity (particularly of the shadows) so that you get the full exposure results you want.
Unfortunately, this is a gross oversimplification of what pros actually do and it does not take into account the fact that there can be color shifts if you have to apply a great deal of post processing on shadow areas.
Do I shoot ETTR? Not exactly. I shoot some variations on that theme that are quite complex and change a bit with different cameras. Moreover, I pursue "optimal data," which is not always exactly ETTR. Some day I'll try to describe the full range of what I do, but it took Ansel Adams several books to tell people what he was doing with negatives and printing, so don't expect a short answer ;~).