While it's true that the mexican goverment supported the Republic, the public was much more divided, liberals supported the republic and conservatives the nationalist movement. Lazaro Cárdenas (mexican president at the time) had a similar ideology than the republicans (agricultural reforms, secularization) and there was a widespread belief that whichever ideology won the civil war was gonna spread to the rest of hispanic america. The biggest support for the republican side came from the mexican sindicates (particulary the CTM) while the biggest support for the nationalist side came from anticommunist organizations.

As for what was the aid, it consisted mostly on rifles and ammunition, the first of such shipments consisted of 20,000 rifles and 20 million cartridges. During the rest of the war, Mexico remained one of the very few countries that supported the Republic with weapons and supplies.