The Mexican League is Mexico's professional baseball league.
It is not affiliated at all with MLB and therefore, is not counted as "AAA" Baseball. I've read that it's level of quality is somewhere near AAA, but I'd guess it is closer to AA and the talent is inconsistent. How many guys go form Mexican League stardom to becoming MLB regulars? Not many.
Teams can loan players to Mexican League teams to get the players more ABs. I believe that the Nats tried to send Ryan CHurch there many winters ago to get him more ABs against breaking balls, but Church declined the invite to Winter Ball.
As for players coming this way, Rafael Martin (for example), had a contract with a Mexican League team. The Nats wanted him and had to make an offer to his MX team to buy his contract rights. MLB and the Mexican teams "honor" each other's contracts - that is, they wouldn't sign a player who is under contract to the other league without its permission.
The Mexican League that is considered AAA is their summer league, not their winter league. I've often wondered why they are considered AAA?