Ans: Mount system call makes a directory accessible by attaching a root directory of one file system to another directory. In UNIX directories are represented by a tree structure, and hence mounting would mean attaching them to the branches. This means the file system found on one device can be attached to the tree. The location in the system where the file is attached is called as a mount point.

Example:-

Mount –t type device dir
This will attach or mount the file system found on device of type type to the directory dir.
Unmount system calls does the opposite. It un mounts or detaches the attached file front the target or mount point. If a file is opened or used by some process cannot be detached.