A thermal flask is actually two vessels one inside the other, but sealed with a vacuum between the two. Because there is nothing between the two layers it is almost impossible for energy (heat) to pass between the contents of the flask, and the air on the outside (if your flask holds something hot, like your tea or coffee) or the other way (if the flask holds something cold, sich as liquid nitrogen, or cold milk). The flask is not perfectly sealed though, as the lid is not vaccum sealed too, so some heat is transferred (escapes, or gets in) that way, and so the contents will reach the temperature of its surroundings, but extremely slower than if it were in a normal open cup.

If you want a more detailed explanation, try the wiki link below. Hope its a bit clearer now