I assume you mean "contiguous free disk space". Hard drives are divided in to blocks of a certain size. When your operating system needs to save a file it is split up into pieces that fit into these blocks.
The system tries to save the file to a sequence of adjacent free blocks because it's faster to find the file again if its not scattered all over the hard drive. Contiguous free disk space is a sequence of blocks large enough to hold a given file.
If your system is giving you an error about too little contiguous free disk space you need to
defrag it or possibly free up more disk space.
