The couplet is the epitaph on a commemorative stone placed on the grave of the Spartans at the site of the battle.
There are numerous translations of the couplet from the original Greek, one of which reads:
Go tell the Spartans, passerby,
That here, by Spartan law, we lie.
The Battle of Thermopylae was the subject of a 1962 film, entitled The 300 Spartans.