Also known as customs fields custom fields are aggregated metadata.
What is a metadata?  According to Wikipedia are data that describe other data. In general, a group of metadata refers to a group of data, called a resource.
The concept of metadata is analogous to the use of indexes to locate objects instead of data. For example, in a library they use tokens that specify authors, titles, publishing houses and places to look for books. Thus, the metadata help to locate data.
Yes, they are data about a data, and we live with them everywhere from the index of a dictionary, through the ID3 of an mp3 (the "Place" where the data is saved as an artist, album, genre) and ending with the EXIF information, which stores information about time, date, camera model and others about the images taken by digital cameras.
Before continuing with the post, it is necessary to clarify that WordPress has by default several general fields among which are, name of post, subtitle of post (post slug), post date, categories, tags and so on.
This amount of metadata is enough for most bloggers, but luckily and since WordPress is a very complete tool, it allows the addition of other fields, depending on our need.
(And that of many plugins that use the custom fields without our direct intervention)
Let's see the example of blissful the I needed to show how much I was running and what I was reading.
Or without going far back, the theme Unstandard due to the special nature of the template makes use of the custom fields, linking 2 images to each post.
As you can see the custom fields are incredibly versatile, and although as a blogger you surely do not use it directly, knowing that they exist can be useful for a future project.
To know more:
I killed her for a yogurt : Custom fields in WordPress.
Yukei: Commenting on some functions for WordPress
Sisifodichoso: Custom fields
Codex WordPress: Using custom fields