Interleave is the process through which gaps are placed between two SECTORS on the platter of a disk.
This was done in EARLIER days when the computers were not quick enough to read continuous streams of data.
WITHOUT interleaving there would be no gaps between the sectors and data would arrive immediately before the reading unit is ready. Due to this to read the same data a complete rotation of the disk would be required again.
The interleaving ratio was not fixed and can be set by the end user depending on their system SPECS. Nowadays the ratio of interleaving is 1:1 i.e. no interleaving is USED.