To bring a formatted partition into the filesystem tree, mounting is required. Here we mount /dev/hda8 to mount point /opt. Mount point is nothing but a directory for a filesystem to attach to. Mount point must be present for mounting to be done successfully. Option -t ext2 tells mount that we are going to mount an ext2 filesystem.
When you want to detach the mounted filesystem due to reasons such as reformatting the disk or fixing the filesystem error on that disk partition, you need to un-mount the filesystem such that the OS can flush the data from cache to the disk to prevent filesystem corruption. To un-mount a filesystem, you need umount. The mount point given the umount must be full path, i.e. the path all the way from / to the mount point.