Your Circuit Playground has some lights that it can use to give you an idea of what it is up to.
These lights, called LEDs (pronounced Ell Eee Dee), are on just about every electronic device you own. Often times they're used to let you know if something is on and if there's an error. For example, here's a cable modem with multiple LEDs.
Each LED indicates the status of the modem. For example, on this device the one to the right is the POWER LED, its lit if there's good power. In the middle is the ONLINE LED, which lets you know if the modem was able to connect to the Internet.
Likewise, the Circuit Playground has two basic (single-color) LEDs: ON and #13
They sit to the left and right of the Micro USB jack
ON LED - this LED will shine green whenever the Circuit Playground is powered. Always check this LED if your board is not acting right, if its flickering or off then you should check your power supply
#13 LED - this is the one LED that you can control. The ON lights up automatically no matter what. The #13 LED, however, is connected to one of the main microcontroller's pins and you can turn it on or off when you start writing code. This LED also pulses to let you know when the bootloader is active and ready to receive a program
It's called lucky #13 because it is connected to pin number 13 on the microcontroller!