Interlacing means that you have multiple experiments on one plate, as is common for QPCR runs. For example, you start pipetting your samples for Gene 1 into the first row of the plate; when you reach the end of the first row, you proceed with the 4th row, because you have two interlacing experiments (Genes 2 and 3) in rows 2 and 3.
Example:
Interlacing layouts are often used to facilitate pipetting of multiple samples.