WordPress at a glance
function is not described

get_tag() WP 2.3.0

Retrieve post tag by tag ID or tag object.

If you pass the $tag parameter an object, which is assumed to be the tag row object retrieved the database. It will cache the tag data.

If you pass $tag an integer of the tag ID, then that tag will be retrieved from the database, if it isn't already cached, and pass it back.

If you look at get_term(), then both types will be passed through several filters and finally sanitized based on the $filter parameter value.

No Hooks.

Return

WP_Term/Array/WP_Error/null. Tag data in type defined by $output parameter. WP_Error if $tag is empty, null if it does not exist.

Usage

get_tag( $tag, $output, $filter );
$tag(int/WP_Term/object) (required)
A tag ID or object.
$output(string)
The required return type. One of OBJECT, ARRAY_A, or ARRAY_N, which correspond to a WP_Term object, an associative array, or a numeric array, respectively.
Default: OBJECT
$filter(string)
Optional.
Default: raw or no WordPress defined filter will applied

Code of get tag: wp-includes/category.php VER 5.1.1

<?php
function get_tag( $tag, $output = OBJECT, $filter = 'raw' ) {
	return get_term( $tag, 'post_tag', $output, $filter );
}