In short: the four foundations of mindfulness (satipaṭṭhāna) are four things to observe: the body, feelings, the mind and principles. The Buddha called them "the path to convergence… to make an end of pain and sadness" (MN 10).
MN 10 in full · What is mindfulness?
Questions people ask
What are the four foundations of mindfulness?
Mindfulness of the body, of feelings, of the mind, and of principles (dhammas), taught in MN 10. Each is observed 'keen, aware, and mindful, rid of covetousness and displeasure for the world'.