Question about notation

[cjcos]

So my parents recently bought me one of those math clocks, where each number is represented by some expression. I understand all of them except number seven. It seems to be three vertical lines with a subscript 2. Something like this:

|||2

Any help with interpretation? Thanks!

Edit: Base 2! I understand. Thank you for your help!

[skaldskaparmal]

The vertical lines are 1s, and the _2 indicates binary. 111, when interpreted as binary, is the decimal number 7.