The phrase "bajar musica gratis Mexicana" translates to "free Mexican music to download." "Bajar" translates to the word lower, which is roughly interpreted to mean download when translated to English. "Musica" is music, "gratis" is free and "Mexicana" is Mexican and when put together means that someone wants to download free Mexican music.
There is another word, "descargar," that means to download, so seeing the phrase used with "bajar" or "descargar" would mean to download free Mexican music.
This phrase is most common for people conducting an online search for free downloads. It may be used when looking for a certain Mexican artist or album.