"Meditations of the First Philosophy In which the Existence of God and the Immortality of the Soul are Demonstrated" (1641) is René Descartes most notable philosophical work (alongside "Discourse on the Method" which precedes "Mediation" and sets its theoretical framework). Descartes' "Meditations" is considered to be the opening shot of western modern philosophy for it establishes its fundamental question such as epistemology and the Rationalist/Empiricist debate.
Descartes' "Meditations" is made out of six chapters, six meditations, one for each day of Descartes' firsthand account of his withdrawal from the world with the purpose of finding an absolute and secure basis for his knowledge of the world. Descartes starts with doubting anything he knows, moves on to finding a reason to be believe he himself exists and finally to proving god exists and therefore anything else.
Here you can find an essential summary of Descartes' Meditations outlining the main arguments and notions of the book. You can also find a detailed account of Descartes' arguments for the existence of God in Meditations.
For a detailed summary of Meditations by chapter/meditation see:
Chapter / Mediation 1: On the things that may be called Into doubt
Chapter / Mediation 2: On the nature of the human mind and that it is easier to understand than bodies
Chapter / Mediation 3: On the existence of God
Chapter / Mediation 4: on truth and falsity
Chapter / Mediation 5: On the essence of material things and the existence of God once more
Chapter / Mediation 6: On the existence of material things and the real distinction between the mind and the body
Each chapter summary contains the main ideas presented in the meditation and the general course of Descartes' thought. The summaries end with a brief explanation of how the meditation relates to all other meditations of Descartes' book.