Just try it out. Unless you're using IE that will answer most of your questions.
It's HTML5. In browsers that don't support this attribute it'll just act like a normal text box.
Max and min are surely obvious.
Placeholder I decided against, it wasn't a good use for that attribute, but in browsers that support it, it just gives a suggestion or hint that disappears when the user enters a value.
Step is the increment that the numbers 'step' by. Most modern browsers will give a spinner that the user can use as an alternative to typing.