Only when all conditions have been satisfied do we reach the command, in which case the form will be submitted.
When the form is submitted - either by hitting Enter or clicking on the Submit button - the (the 'value' of the field called 'input' belonging to the form). In a real-life situation you will most likely have more fields to check, and more complicated conditions, but the principle remains the same.
Other form values are available using a similar syntax, although this becomes more complicated if you're using SELECT lists, checkboxes or radio buttons (see below for examples). All you need to do is extend the command is encountered, execution of the function is halted.
Normally you would modify this so that you could submit or not submit the form based on the number of items selected. For example, a text input that only needs to have a value if a checkbox is checked: Using simple logical operators and the functions supplied above you can do all sorts of client-side form validation.
To take things a step further you can even explore Ajax Form Validation which lets you supply real-time feedback using server-side scripts triggered by Java Script events.