Descartes set out to find something he could not possibly be wrong about. His method was to doubt everything that admitted any doubt at all: the senses deceive, so the physical world goes; dreams are indistinguishable from waking, so the body goes; a sufficiently powerful deceiver could make even arithmetic seem certain while being false, so mathematics goes. What is left when everything doubtable is discarded?
One thing, he argued. The doubting. However radical the deception, someone is being deceived — and that someone must exist to be deceived at all. The very act of questioning your existence performs the thing it questions. This is the cogito, and its force lies in being self-verifying: you cannot coherently think "I do not exist," because the thinking would be doing the disproving.
An important subtlety is often lost. Read as a syllogism — everything that thinks exists; I think; therefore I exist — it needs a suppressed premise it has not earned. Descartes indicates that he meant something more immediate: an intuition grasped in a single act, where thinking and knowing one exists are not two steps but one. That reading is more defensible, and it is why the Latin is usually left untranslated as a single unit.
The objections are as famous as the claim. Lichtenberg argued Descartes helped himself to too much: the evidence supports "there is thinking," not "I think" — the enduring self is smuggled in, a criticism Hume later sharpened by reporting that when he searched his own experience he found perceptions, never a self having them. Others note the cogito is punctual: it secures existence only in the instant of thinking, giving no bridge to a self that persists between thoughts. And its real trouble is what comes next — Descartes' route from this single certainty back to a reliable external world runs through arguments for a non-deceiving God that few philosophers have found convincing.
So the sentence is both less and more than its reputation. Less, because it delivers a very thin conclusion. More, because it reframed philosophy: after Descartes, the mind's own contents became the starting point for knowledge, and modern epistemology has been arguing about that inheritance ever since.