Ravin answered that I could have undermining. WHAT'S THAT?

I asked about a big scary hole in the planter next to my garage. There are no rain gutters up, so that part makes sense now, but, Ravin, what is undermining? I have been a homeowner for all of six months, and everything about it is scary! The new walkway and landscaping were already in place when we purchased our home. I am going to start with fill dirt.