[p], [t] & [k] are aspired sounds
It means they are pronounced with a expulsion of air. Place your hand before your mouth —if you are pronouncing them correctly, you should notice a small amount of air coming from your mouth.
[p], [t] & [k] are aspired when they are in a stressed syllable, which generally speaking would happen at the beginning of a word.
However, the presence of [s] before [p], [t] & [k] blocks aspiration.
When [p], [t] & [k] are in final position in a word, aspiration is optional.