The Upanishads are a series of ancient Hindu sacred scriptures composed in the Sanskrit language thousands of years ago. They are a part of the Vedas, the core scriptures of Hinduism. Specifically they are the main section that explains the esoteric, inner meaning of the Vedas, and are concerned much less with outer rituals and more with internal mystical experience of the One Reality that encompasses all existence. The Upanishads primarily use the term Brahman to refer to this One Supreme Reality.
Being the esoteric part of the Vedas, the Upanishads contain some of the central philosophical-spiritual concepts of Hinduism and ancient Indian thought generally, and are among the most important literature in the history of India and indeed the whole human race by any reasonable standard, even if you’re not a Hindu.
The central idea of the Upanishads is that the consciousness of every living being is the Atman or Self, sometimes loosely translated as soul, a primordial unit of consciousness independent of and ultimately unaffected by the body; and furthermore that the Atman at its root is identical with Brahman, the Universal Consciousness. This identity with the Universal Consciousness or Ultimate Reality can be known experientially, not just intellectually (indeed it cannot be known with intellectual certainty until it has been known experientially), and this is the supreme experience, being characterized by infinite Anandam or spiritual joy. The Upanishads are essentially manuals for attaining this experience.
They contain many more things too; indeed so many that it would take a vast treatise just to comprehensively summarize their contents, but what I described above is the main point.
As for the word ‘Upanishad’ itself, it means ‘to sit near’, and refers to the Vedic student sitting near his or her Guru who tells them the teachings that were later collected in the Upanishad texts.