Your Arduino 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 middel is the ONLINE LED, which lets you know if the modem was able to connect to the Internet. Between those two, thereis the SEND and RECEIVE LEDs, that blink when you upload or download data through the modem.
Likewise, the Arduino has four LEDs: L, RX, TX, and ON
On the UNO, three are in the middle and one is to the right
On different Arduino variants, the LEDs may be in a different location. Like on this Metro, they're all on the left in a row
The four LEDs include three 'automatic' LEDs and one 'user controllable' LED: