There are three definitions for pilas:
1. A water deposit with an area for washing (click here for pictures of modern pilas). The one in the picture is a communal pila. It has 5 spots for 5 women to come and wash. Women would come here wash and gossip making I am sure a wonderful event. Communal pilas are still in the rural areas. Pilas are the next step above washing clothes in the river on a rock.
You can find a pila at basically every house in Guatemala
2. Batteries. That toy comes with batteries included. El juguete trae las pilas incluídas.
3. Colloquial expression: From the concept of batteries comes this very common expression used when somebody needs to get more proactive or needs to be more energetic. One would say: "Ponete las pilas." or "Me voy a poner las pilas." (Put on the batteries. or I will put on the batteries.) Also someone that is pilas is a person that is energetic, thinks quick on his feet, similar to chispudo.
The picture is a communal pila in Finca Filadelfia
Thanks to Pablo`s comment for reminding me to write another very Guatemala memory. At soccer games the commentator would switch the microphone as follows: Luis was commenting on a play and to give the microphone to the other commentator he would ask: ¿José cual es la Pila? (What is the battery) Jose answers "Ray-o-vac- tremenda en duración"(Long lasting Ray-o-Vac) and keeps commenting.