What is post_max_size
Typically this is set much higher however as an example, suppose your max_upload_file size is 8MB and in a page you are attempting to use images and perhaps a form with text. So if you have 3MB of text and 6MB of images your post size becomes 9MB which is more than your allowed post_max_size. Normally you want to keep post_max_size and upload_max_filesize the same.
What is upload_max_filesize
This is the size which is allowed to upload files. If upload_max_filesize is set to 150MB this means you cannot upload files larger than 150MB.
How to Modify the Values
MultiPHP INI Editor is included as part of cPanel and allows amongst other values, the post_max_size and upload_max_filesize to be modified.
Once the value has been updated, click the Apply button at the bottom of the page.
To further confuse things, each application may have a way to modify their internal file upload size. For Omeka it’s the config.ini file. This is covered in a separate Help Article.