What does backwash mean? I mean, I know what it means, but I can't really explain it.

The fluid, known as backwash, occurs when eating or drinking; in short, the material being eaten or drunk is returned to its container.  Usually this is accompanied by the bacteria (and perhaps other foods) from the mouth of said individual.  Indeed, it is usually the foreign agents (elements not part of the original food or drink) that is used to identify the presence of backwash.
This is mildly confused by the fact that backwash can refer to both the material being returned to the food or drink or can refer to the process by which said foreign agents return.