Metafields are what you are used to on the other CMS’s as custom fields.
Metafields allows you to add and store custom data related to a resource type and display it afterwards wherever you want in the theme.
You can display them in the theme via the theme editor or when you need something more custom via the theme code.
On Shopify you can have 200 metafields for each resource type.
What does this mean?
That means that for each of the following resource types you have:
Products – 200 metafields
Variants – 200 metafields
Collections – 200 metafields
Customers – 200 metafields
Orders – 200 metafields
Pages – 200 metafields
Blogs – 200 metafields
Articles – 200 metafields
The number of metafields are not increased or decreased based on your Shopify subscription, that means that each subscription receives the same number of metafields.