Where the composer requires a sounding staccato, the clarinettist plays "TA TA TA". Where the music requires secco (Italian for dry) the player plays "TAT TAT TAT". With the first the reed and the air column reed have a chance to swing on a little, with the latter the tone is muted instantly. If you are quick with your tongue, this may not sound pretty (and that may be what the composer has intended when asking for "secco". It is important to understand that most other instruments can not produce such a short and sharp staccato, so when playing together with eg. strings, you must adapt to what they do play. Maybe you don't take too literally what the notes say.