The 't sound' /t/ is unvoiced (the vocal cords do not vibrate while producing it), and is the counterpart to the voiced 'd sound' /d/.
To create the /t/, air is briefly prevented from leaving the vocal tract when the tip of the tongue presses against the tooth ridge while the sides of the tongue press against the upper side teeth. The sound is aspirated when the air is released. The aspiration for a /t/ is greater than the aspiration for a /d/, especially when it is the first sound of a word or the first sound of a stressed syllable.
Advanced ESL/ELL pronunciation students should be aware of /t/ allophones, of which there are many. The /t/ is usually only pronounced as described above in the following circumstances: