Means three in the ternary (base-3) numeral system.
Means four in the quaternary (base-4) numeral system.
And so on...
One exception is the unary (base-1) system, where 10 probably means "WTF? ^^".
Computer: 10 is two!
Octopus: 10 is eight!
Modern human: 10 is ten!
The Hitchhiker's Guide to the Galaxy: 10 is thirteen!
Haxxor: 10 is sixteen!
God: 10 is infinite!
When used about someone else, it is a rating of the highest appreciation
Friend: haha okay
Person: wooaa look at her
Friend: yeh i know, she is a 10