"You shall not cook" — the Torah explicitly prohibits cooking meat in milk. But what about eating the cooked mixture? The verse says "cook," not "eat." Does the absence of an explicit eating prohibition mean eating is permitted?
The Mekhilta answers through an a fortiori argument. Consider the Passover offering. The Torah does not prohibit cooking the Passover lamb in a certain way — but it does prohibit eating it raw or boiled. If something that is permitted to cook can still be forbidden to eat, then something that is forbidden to cook must certainly be forbidden to eat as well.
The logic is airtight: cooking meat in milk is a more severe prohibition than cooking the Passover offering in an unauthorized way. If the lesser prohibition (unauthorized cooking of the Passover) generates a ban on eating, the greater prohibition (cooking meat in milk) must also generate a ban on eating. The eating prohibition follows automatically from the cooking prohibition through logical necessity.
This derivation is important because it shows that not every prohibition in the Torah needs to be spelled out explicitly. Some prohibitions are so logically necessary that the Torah relies on human reasoning to fill in the gaps. The cooking ban is stated. The eating ban is derived. Together they form a complete prohibition that covers both preparation and consumption of the forbidden mixture.