ΑΥΡΗΛ Γ´ ΣΤΕΦΑΝΗΦΟΡΟΥ
Αὐρηλ(ίου) γ´ στεφανηφόρου

For Aurelius, who won the prize for the third time.

It's a commemoration of someone who won a prize in an athletic contest. Aurelius was a common name in the Roman era; there are at least 13 other inscriptions from Magnesia mentioning various Aureliuses, and at least 7 inscriptions commemorating prize-winners.

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Edit. I can see some rather misleading answers elsewhere in this thread, so perhaps I should make some further clarifications:

1. Αὐρ. would be the usual abbreviation for "Aurelius" in inscriptions, but Αὐρηλ. is not unheard of;
2. στεφανηφόροι "prize-winners" are routinely commemorated in inscriptions, often with a numeral to indicate how many times they won (e.g. I.Kaunos 137 Ποπλίου Οὐηδίου | [Κ]απίτωνος στεφανηφόρου β´ καὶ χει[λιάρ|χ]ου λεγεῶνος δ´, "...Publius Vedius Capito who won a prize twice and was military tribune of the fourth legion...")
3. There are cracks in the stone that have misled another poster into reading ΑΥΡΗΛ Γ´ ΣΤΕΦΑΝΗΦΟΡΟΥ as ΑΥΡΗΑ Γ´ ΖΤΕΦΑΝΗΦΟΡΟΥ, but (a) they're just cracks, (b) ΑΥΡΗΑ doesn't mean anything, and (c) ΖΤ isn't a possible letter combination in ancient Greek.

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Second edit, four weeks later. On reflection, I don't buy Αὐρηλ as an abbreviation for "Aurelius". Αὐρ is the standard abbreviation. Αὐρηλ, then, must be "Aurelianus".

Last word

ΣΤΕΦΑΝΗΦΟΡΟΥ?

Edit: Stephanephoroi ('the one who wears the crown')

http://fr.wikisource.org/wiki/L’Encyclopédie/1re_édition/STÉPHANEPHORE

Google Translate:

STÉPHANEPHORE, sm ( Asian antiquity .) στεφανηφόρος ; we nommoit in antiquity stéphanephores , some priests and individuals pontiffs of a distinguished order, which wore a laurel wreath, and sometimes a golden crown, in public ceremonies. This priesthood was established in several cities of Asia, Smyrna, Sardis, Magnesia Meander, in Tarsus, and elsewhere. By the monuments we see that this dignity was annual eponymous & in some cities. The stéphanophores formerly devoted to the ministry of the gods, then tied to the same cult of the emperors. We read in an inscription Tiberius-Claude Sardis, had been stéphanephore , ΣΥΡΑΤΗΙΟΥ. ΔΙΣ. ΚΑΙ. ΣΤΕΦΑΝΗΦΟΡΟΥ ; but we do not know if he was pontiff gods or emperors.

We also nommoit stéphanephore the priest who was at the head of women in the celebration of Thesmophoria. But we nommoit ultimate stéphanophore the first pontiff of Pallas, like Hercules bore the name of daduchus . Potter, Archæol. Greek. tom. I. p. 206 . ( D. J .)