Why is it advised to put a Styrofoam plate under an Erlenmeyer flask?
You provided the additional information in the comments that your experiment was about measuring the change in temperature that occurs upon mixing. This should give you a hint as to whether the substances mix with water in an endo or exothermic manner.

When measuring temperature changes it is crucial not to influence the temperature of the reaction mixture in any way. Naturally, that would for example mean not to keep the flask in your hand (body temperature is typically more than room temperature). But some error sources are not as easily visible. In your case, the table top was an error source. Whatever the material, it likely conducts heat better than air — meaning that if your flask was on the table a significant portion of the heat generated (or used) would be conducted away (or onto site) by the table resulting in a lower change in temperature. That is why these experiments are typically performed under maximally insulating conditions. One of the best and widely available insulators is polystyrene because of its high air content (air is a good insulator). Hence, keeping polystyrene under a flask reduces the error caused by the table conducting heat.