This sentence stands for the imperative mood. Sentences standing for this mood are commands. In fact, this is a negative command because it uses the word No at the beginning of the sentence. Comas is the conjugation of the verb comer for the second person singular tú in the imperative mood in negative form. On the other hand, pastel means cake. So the sentence:
No comas ese pastel means Don't eat that cake
Moreover, in affirmative form the command is:
Come ese pastel meaning eat that cake
So the conjugations vary according to positive or negative form.
