It’s how many times can you send (writes) or get (reads) a piece of information to and from where it’s stored. It’s not how much can be stored, or move at once, or how long each read or write takes.
When your computer is slow, this can be the reason. If it cannot do any more reads or writes, but needs to, the work piles up. But it can take a lot of money to store things, so buy enough but not too much. It helps to know how much you’ll need before you buy, or to buy something to grow with you.
Imagine a trip to the book-sharing building. The books can be any size, but the people there say you can't take more than five books home. If you need to read seven books, you can only do five, then line up the next two. You might be able to bring back many books, say, everything your family took home all yourself, but now the most books might be how many you can carry, not how many you’re allowed. Small books might be easier to carry back, so maybe you can bring back more at once, but all we watch here is the count of books at a time. Reads and writes for storing information act very much like this. Lots of things can change your counts, so while you might use the counts to check your work, it’s just one of many things to watch.