Blocks of similar experimental units
The differences between the experimental treatments can be estimated more accurately if the experimental units can be grouped into blocks of similar units.
A randomised block design is equivalent to a separate completely randomised design within each of the blocks.
In the simplest randomised block designs, all block sizes are all equal, and all treatments are used the same number of times within each block.
Don't forget to randomise allocation of the treatments to experimental units within each block.