What is deposition of matter in science mean?

It means that matter i.e. certain compounds or substances bundles above that particular substance. Like for example you can say deposition of matter on your coins. Here the deposition of the green substances on the copper coins is what is matter. That green substance is due to the formation of copper sulphate. Coins are made of copper (certain). Due to atmospheric sulfur dioxide the copper gets oxidized. Thus copper sulphate accumulates on the surface of the coins and leads to its deposition.