In simple terms these are good and bad thoughts.
Good thoughts would lead us to improve our lives. Bad thoughts would lead us to slothfulness and eventually ruin our lives.
Why these thoughts just pops-up in our minds?
It often times occur when we're at rest, or at peace or maybe during fasting or prayers or meditations when we empty ourselves.
These thoughts just came out of nowhere.
@chbartist explained it very well. I just explain it in my personal view and understanding.
In my experience, @chbartist is true that we have to confront these thoughts and distinguish with solid valid reason which are good thoughts and which are not.