Straws work because of the air pressure. Air pressure is the weight of air pressing down on earth and all other objects present on it. Now, when the straw is in the liquid, the air pressure present on the liquid and straw is similar, causing the levels of liquid to be equal. However, when we drink the liquid through straw, we first inhale causing the air from the straw to flow into our lungs. Now, due to the absence of air, the air pressure in the straw decreases. Hence, the air pressure present on the liquid of the glass pushes the liquid into the straw(diffusion), causes it to move upwards and thus helping us drink.
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