Memory usage in WordPress refers to the amount of memory used by a PHP script. This script could be a theme, a plugin or a core WordPress file. A well written WordPress plugin or theme will attempt to reduce its own memory usage and not cause heavy load on your server by using excessive memory. PHP scripts consuming too much memory can also slow down a website. Many shared web hosting service providers have safeguards in place to automatically kill PHP scripts that are exceeding more than an allocated amount of memory, resulting in website downtime, and sometimes even suspended hosting accounts. To optimize memory usage in WordPress, it is strongly recommended that you use a caching plugin which reduces the load on your web server by serving cached copies of webpages.
In many cases you can see for yourself how much memory the server you are hosted on is using. In cPanel, you can click on “Service Status: Click to View” at the bottom left of your cPanel log-in page if your host allows you to see such things. Knowing what the normal usage is will help you to spot when things are abnormal and possibly a risk to your website. The memory usage is shown as a percentage of total memory. Other hosts who do not use cPanel may have similar options available to view memory usage other than through fatal errors that are displayed when too much memory is being used. If needed, you can also ask your host to increase your allocated memory when running scripts or plug-ins that require them, although this is not recommended unless you know exactly what you are doing.