Franeleros are professional hoggers of parking spots on the streets that surround office and entertainment districts. They usually get to the spot before sunrise (in the case of office districts) or sunset (in the case of entertainment districts) and locate crates (huacales) on the street to prevent drivers from parking. Then, when a potential parker shows up they ask for a fee that usually includes a mandatory surcharge for a half-ass car wash.
The word franelero comes from the fact that they wave at drivers with a piece of flannel when they lure them to take a parking spot and when they help them park (echando aguas).
The editors of this blog recommend to play along with these dudes and do not incite them to any retaliation (by forcing them to move their huacales or not paying the fee). Otherwise you might find your car scratched, on bricks or you might not find it at all, when you come back.
The image below shows the discontent of some business owners with franeleros that not only charge a fee but require drivers to leave the keys.