Base 10 means the way we normally calculate with 10 following 9 and 100 following 99.

Base two (binary) is the way computers calculate, with 10 following 1 and 100 following 11.

Type 2 instead of 1 in the "value to convert" box, and click on "calculate" to see what 2 is in binary. Then try it for 3, 4, 5 etc.