Goblet cells are single cells that make and release mucus. They are named for their shape: a narrow base and a wide top packed with mucus granules, a bit like a wine goblet. They sit scattered among the ordinary column-shaped cells that line mucous membranes, especially in the small and large intestine and the airways.
OpenStax Anatomy and Physiology describes a goblet cell as a mucus-secreting unicellular gland and classifies it as the simplest kind of exocrine gland, meaning it releases its product onto a surface rather than into the blood. The mucus they produce coats and lubricates those linings and helps trap particles and microbes before they reach the tissue underneath.
Source: OpenStax Anatomy and Physiology 2e, 4.2 Epithelial Tissue
Image: goblet cells in simple columnar epithelium by Laboratoires Servier (Servier Medical Art), CC BY-SA 3.0, via Wikimedia Commons.