The common story is “he ran from Marathon to Athens to alert Athens of the Greek victory at the Battle of Marathon, and promptly died after doing so”.

The reality is that before the battle, he ran the much longer distance from Athens to Sparta over 2 days to ask for Spartan aid in the upcoming battle. Sparta said they would help, but could not leave for a few days due to religious reasons (I’ve seen some sources cite a religious festival, and others say it was because they could only set out under a full moon). So Philippides then ran back to Athens to tell them. Sparta would not arrive until after the battle, but Athens won anyway.

There are some claims that then Philippides ran back to Athens as per the common story, but these are attested much later and so are likely untrue.