Cognitive distortions are simply ways that our brains convince us that something is true that really isn't. These distortions in our thinking are often replays of past patterns that we've learned, often in our developmental years.
Unfortunately, such thinking often causes us to then feel badly about a situation or ourselves. It becomes a bad cycle.
A common cognitive distortion is All or Nothing Thinking where we look at things as strictly black or white. Such interpretations are often distorted from reality and can result in intense negative feelings.
In comparison, if our interpretation wasn't so strictly black or white, our resulting feelings can be more moderate and likely easier to deal with and have less lasting effect.
As an example, a co-worker says something to you that you interpret in an all-or-nothing way. The next thing you know you're upset and acting in an extreme manner and actually causing a conflict with your co-worker that wasn't even there to start with.
A big first step in stopping cognitive distortions is to recognize when they're happening and acknowledge them. For example, to say to yourself something like, "Oh, I'm thinking of this in an all-or-nothing manner. Such thinking may not be accurate and can result in unnecessary negative feelings. The reality may actually be somewhere in the middle." Then possibly take a break from thinking about the topic until your emotions have eased, then revisit it. Often doing so allows you to see it in a less all-or-nothing light.