Literally, "Granny Crap". Generally, a woman, elderly, in charge of hygiene of a toilet, and in charge of charging you for using it. The amount may go anywhere between symbolic and almost outrageous, and is usually unrelated to the actual cleanliness of the toilet (ahem, restroom).
The institution of a baba Sera (from "srati", to shit) is very old, of unknown age and origins, and is on its way to either oblivion or uplifting of social status. In some places, they stopped charging. In others, they probably elevated the position to one having some kind of fancy uniform instead of the traditional worker's blue smock, more cleaning devices than just a hose and a mop.
In the local lore, the cult movie "We aren't angels" has a scene where the main character, on his downward spiral, wakes up in the bed of a baba Sera. Not believing his eyes, he asks "um... did we...". She nods happily: "three times!".