The Spanish phrase "trailas gratis" translates to "free trailer" in English. The word "gratis" means free or complimentary, while the word "trailas" is a slang word for trailer.
The word "trailas" is only used in northern Mexico and the American Southwest. In this case, the English word "trailer" is spelled and pronounced according to Spanish conventions. This is an example of an Anglicism, or the use of an English word in another language. Near the U.S.-Mexico border, Anglicisms are very common due to the prevalence of both languages. Elsewhere in the Spanish-speaking word, a trailer is referred to as a "remolque."