The simplest way to distinguish a computer from a calculator is that a computer has memory, the ability to store and then later retrieve information. This storage is called memory, and every piece of PHP script in WordPress uses a certain amount of memory whether it’s a script that’s part of a theme, a plugin, or the core WordPress files.
Every time a PHP script is executed, it must be delivered via the server. Therefore, the more memory that is being used, the slower the server can deliver the information. Some shared web hosts impose limits on the amount of memory that PHP scripts can take up, shutting down the site (or even suspending the account) if these limits are exceeded.
Note: If you’re using a WordPress site hosted by WordPress.com, then you don’t have to ever worry about memory usage because you are unable to add new PHP scripts.
If you’re curious about how much memory your WordPress site is using, you can use your web host’s administration software (often called cPanel) to check this information. For cPanel, click on the words “Service Status” that are usually written in the bottom left-hand corner of the admin screen.
Usually, the way that most WordPress site owners get into trouble with using too much memory is by running too many plugins or installing plugins that take up too much memory. If you find that your website is using up a lot of memory, try disabling plugins one by one and then seeing how this affects the amount of memory you are using. Themes, too, can also be the cause of bloated scripts that use up a lot of memory.
You may, however, find that your site is working flawlessly, but you just need more memory. If so, contact your web host and see about having your allocated memory usage increased.