Overthinking means unproductive thinking. I don't like the term "overthinking" because it implies, incorrectly, that a certain amount of thinking is the right amount (what amount? never specified) and to go above or below it is irrational. This is no help at all. Sometimes people who are threatened by what you think, even if you have a good point or ask a valid question, will tell you not to overthink. This is dishonest on their part and you need to be careful. However, there IS such a thing as nonproductive thinking. Here are some of the characteristics of nonproductive thinking:

1) Thinking the same thing over and over again, without attempting to reach a solution;

2) Thinking that does not lead to a solution, or at least closer to one;

3) Thinking that paints you as a helpless victim without integrating in what you DO have control over;

4) Thinking without reference to action, or thinking never followed up with action (even experimental action, to see what's true and what isn't).