Awareness is a general term for how a character senses the world around them. This includes the obvious senses such as sight and hearing but may also include psychic powers or the ability to sense electrical currents (as some sharks can). Exactly which senses are used depends on the character and the situation.
When a character wants to know more about the world about them, such as "Is there a monster behind that door?", they make an awareness action.
As with any action, the character must play appropriate cards. Often these will be ability cards such as Alert and relevant environment cards like Woods & Forests. Some settings may have Super-Hearing or X-Ray Vision life cards. In addition to any cards, characters can use flow as a pseudo-card for an additional 1d6+0. For many characters this will be all they have to use.