Polynomially related lengths under two different encodings
Garey and Johnson are referring to the fact that any encoding scheme for some instance $I$ of a problem $\Pi$ will only differ in length (i.e. number of bits) by a polynomial amount. For example, consider two possible ways to encode a graph: adjacency matrix, and adjacency list.

It is not possible to obtain a super-polynomial speedup by using one encoding over another for various instances of problems. Again, this notion relies on the fact that encodings are reasonable. That is to say, we are not unnecessarily padding our encoding with some "junk" information.