The direct Spanish to English translation of "musica para celular gratis" is "music for cellular free," as stated by spanishdict.com. It means that the cell phone user can download free music or ringtones for their phone.
A more natural translation of the phrase is "free music for cell phones." Another way of saying it in English is simply, "music for free." This phrase is likely on a Spanish language webpage featuring free music downloads, such as mp3xd.com. The music may be all Spanish or a mix of Spanish and English language songs for users to keep on their cellphones or other devices.