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.

Signature

get_tag( $tag, $output = 'OBJECT', $filter = 'raw' )
tag
 (int|WP_Term|object) A tag ID or object.
output
 (string) Optional. 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.
Default: 'OBJECT'
filter
 (string) Optional. Default is raw or no WordPress defined filter will applied.
Default: 'raw'

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.

Source

function get_tag( $tag, $output = OBJECT, $filter = 'raw' ) {
	return get_term( $tag, 'post_tag', $output, $filter );
}
WP Trac GitHub

Link here