From a book: DESCARTES KEY PHILOSOPHICAL WRITINGS.
In all these operations this cognitive power is at one time passive, at another active, and resembles now the seal and now the wax. But the resemblance on this occasion is only one of analogy, for among corporeal things there is nothing wholly similar to this faculty ..
Descartes is a careful thinker and the quotation is a miniscule part of a much longer discussion. So this this only be a rough suggestion of what he is writing about.
Wax is malleable material that takes impressions. The seal is the stone or metal device which is used to make an impression on the wax. Descartes is saying that the cognitive process plays both roles -- at one point it operates to make an impression (you figure things out and store the information in your mind) at another point it is the wax (it functions as memory, it absorbs the impression and retains it).