"I think, therefore I am."
This is a translated version of a philosophical statement written by René Descartes: "Je pense, donc je suis". You also often see it in its translated latin form, "Cogito, ergo sum". It first appears in his Discourse on Method (Discours de la méthode, in french) in 1637, and later again in his Meditations on First Philosophy (this time in latin), where he expands on the concept and refines the actual statement to something more accurate (Ego sum, ego existo, something like "I think, I exist").
Saying "I think, therefore I am" does not mean that you value intellectual activities above other things. It does not mean that you strive for a better society. Saying this phrase does not make you an educated, cultivated, sophisticated person.
You're not René Descartes, and you're not thoughtful.
What "Ego sum, ego existo" means is simply the first thing that you can be assured is true, before absolutely anything else. Long story short, imagine that all that you know, is false. You aren't sitting in front of a computer right now, you aren't really breathing, in fact maybe you don't even have a body. Forget everything. Everything you see, hear, touch, smell, and taste, are perceptions that you cannot 100% prove to be true***.
However, even if all those things were to be false, it does not take away the fact that you do feel them. You do think you are sitting in front of a computer reading this. You do think that you have a beating heart, that you are a human being, and all that. What does this mean? There are two possibilities: either your perceptions are true, or they are false. In the former case, no problem; you know that you exist. In the latter case, something, whether it is some supernatural being or maybe even your own subconscious, is tricking you into feeling all of these perceptions. But the fact that there is a deceiver means that there is a deceived. You think your reality, so even if you are deceived, one thing remains undeniably true: you exist; otherwise, there would be nothing to be deceived. As long as you are thinking, you can be assured that you, in fact, exist.
Get it now? Stop misusing the sentence!
*** Sounds familiar? It should. The whole premise behind the Matrix is built upon this reflexion of Descartes. Obviously a very simplified version; even outside the 'dream world', humans are still humans. But still, I think that was the reason the first movie was so successful in the first place.