What is a backlog?
Your "backlog" is the pile of games you've bought but never started, or started and never finished. Buying games during a sale is far easier than finding time to actually play them, which is why almost every Steam library has at least a few titles sitting at zero hours.
A backlog isn't inherently a problem. The issue is when it keeps growing and starts creating guilt. SteamPersona's backlog analyzer measures your unplayed-game ratio and activity level so you can see, in concrete terms, how large that pile really is.
Practical steps to shrink it
The most effective approach is sorting your library by playtime and putting the zero-hour games on a visible list. An invisible problem doesn't get solved. Making it a rule to finish one backlog game before adding a new one during a sale directly slows the pile's growth rate.
Starting with short, finishable games keeps motivation up; finishing a 3-5 hour title between huge open-world games shrinks the list in a way you can actually feel. Hiding games you're genuinely no longer interested in (using Steam's own hide feature) also reduces the mental weight of the list.