Fixed Income?

I know I can be simple (aka...stupid) but can anyone explain to me what a "fixed income" is? My dad always jokes that now that he and my mom are retired they are on a "fixed income." What I really want to know is, who isn't on a fixed income? This comes up because I was watching the news the other day about a gas station that had an 8 hour wait for gas because they were practically giving it away. They interviewed a woman who said "I'm on a fixed income, that's why I'm here." Well shit, who isn't? Isn't that one of the reasons people can't afford their houses/groceries/gas etc? It's not like the Army says "hey, gas prices have gone up so we'll throw a little extra cash your way" Is it really about earning potential? In that case, why retire? It's not like prices are ever going to drop, I certainly don't see gas prices going back to $2 a gallon.
That being said, our gas prices have dropped almost 20 cents! How about everyone else? I was excited to see that yesterday, however I anticipate that when I get out today they will have jumped 50 cents.
I attached a video from Myrtle Beach when we went miniature golfing. Kids crack me up. They all do exactly the same thing (even though we all think our own kids are the funniest/smartest/cleverest). I forgot that I have to hold the camera straight, but it will flip upright pretty quickly.