A “hbox” in TeX is a horizontal box. In 99% cases when laying out text, it’s a line of text. “Underfull hbox” means “I couldn’t stretch the content of this line far enough, so it will look janky as f due to the increased spacing”. “Overfull hbox” means “Well, I tried my best to hyphenate and line-terminate, but this word will stick out of the margin and will look stupid as f.”
Most of the time this is caused by a word that auto hyphenation can’t deal with. You need to add a manual hyphenation exception. I can’t remember how to do that, sorry, because it’s been a while and also I’m mildly drunk, sorry.