Code Reference is what the name implies; a code reference for WordPress. This tool allows anyone to search for code inside of WordPress, whether it be functions, classes, hooks, or more.
The function the_date() is a function that retrieves and/or displays the date the current post was written. According to WordPress, the_date() "Will only output the date if the current post's date is different from the previous one output."
This function has four parameters: $format, $before, $after, and $echo. As listed on the WordPress reference, these are their functions:
$format: (string) (Optional) PHP date format. Defaults to the 'date_format' option. Default value: ''
$before: (string) (Optional) Output before the date. Default value: ''
$after: (string) (Optional) Output after the date. Default value: ''
$echo: (bool) (Optional) Whether to echo the date or return it. Default value: true
WordPress Code Reference is a great place to get an idea of what you're working with in WordPress and how to access and use the various functions and bits of code available to you. Along with tutorials on its use, it also provides the source of the function inside the WordPress files.