The block size is the unit of work for the file system. Every read and write is done in full multiples of the block size.
The block size is also the smallest size on disk a file can have. If you have a 16 byte Block size,then a file with 16 bytes size occupies a full block on disk.
The book "Practical file system design" states:
Block: The smallest unit writable by a disk or ﬁle system. Everything a ﬁle system does is composed of operations done on blocks. A ﬁle system block is always the same size as or larger (in integer multiples) than the disk block size.