You are trying to perform an operation that references one or more lists or matrices whose dimensions do not match. For example, trying to multiply a five-element list by a two-element list will produce this error because the number of elements in the lists do not match.
If you're getting this while trying to graph a function, it's probably because the calculator is expecting stat plot features. You can correct this by turning off the stat plot features. Press "2ND" "STAT PLOT" and then select 4:PlotsOff.
