A mount is a piece of equipment connected to your telescope that moves where it is pointing to track the movement of objects in space as the Earth rotates.
This allows you to focus on an object and track it for longer and is essential for astrophotography as it allows you to focus on what you are imaging for longer. This means you can gather more light and take better pictures.
This is especially important for deep-sky photography (galaxies, nebulae, etc) where you are taking pictures of things that are extremely far away and you need time to gather the light for the image.