Blinking light

I just received my arduino UNO and started to download the software to it. When I plugged it in to my computer I immediately got a flashing light.
I know nothing about arduino and little about electronics.
I thought I needed to download a sketch for the blinking light?
I'm working on a digital readout for my mill
Thanks.....dan

They typically install the Blink sketch when they install the bootloader. When you apply power the Arduino will run whatever sketch was last loaded. When you upload a sketch it will overwrite whatever sketch was last loaded.

Thanks John ...so when I upload the readout sketch it will delete whatever is in the arduino? In other words the arduino only accepts one sketch at a time.
I might be getting too old for this stuff but it's fun learning new things just the same. Dan

Maineshops:
Thanks John ...so when I upload the readout sketch it will delete whatever is in the arduino? In other words the arduino only accepts one sketch at a time.

Yes.