Caching is when data from a website is stored in a temporary storage. This allows a user to come back to the site and the data can be quickly and easily accessed.
You may be aware of it more when you think about cache on your phone.
Every website or app must have some data downloaded for you to be able to view it. Caching is the process where that data is stored so you do not need to download the data twice. This should allow the site to load quickly.