This constant is used to define the minimum size allowed for user logins. By default, its value is 4, which forces the minimum login size to 4 characters.
As for all Variables for customising SPIP of SPIP, it is possible to customize the value of this constant, in your config/mes_options.php (see the article dedicated to it).
For example, you will allow users to use logins of at least 3 characters by setting the value of this constant to 3.
// allow authors to choose a 3-character login
define('_LOGIN_TROP_COURT', 3);
You can also control the minimum length of passwords with _PASS_LONGUEUR_MINI.