what is entropy in terms of disorder?

In terms of chemistry- entropy refers to disorder, so basically things coming out of order. So a reaction involving chemical A moving from a solid to a gaseous state. This change in state of matter, from a molecular perspective has shown that the molecules are not in a organised state and have started to move away from each other and become more chaotic, or disorganised. Hence the entropy has increased.

This analogy may help. Say you have a folder with notes perfectly organised by date, all neat and protected in plastic wallets. You could say that the folder is very organised (or has small entropy). Over time though through revision and carrying the folder around, the folder will become damaged here and there. Some notes may become creased and the notes will not be perfectly in order. Eventually over time the folder becomes more disorganised- entropy increases. It is this example which illustrates the key thermodynamic principle which governs not only chemical reactions, but also a lot of fundamental laws in science. To organise the folder again, you would have to actively arrange everything- i.e. put energy back into the system to make it organised.