Array: What is this value based on?

Question

I need to do an import of an array of checkboxes, but there is a value that I don't how it is determined.

The array is based on a user meta option that I created that shows a list of category checkbox options. It grabs all of the categories and creates a checkbox option that can be saved to a user's profile.

This is part of the coding I used to create the user meta option:

$args = array( 'hide_empty' => 0, 'taxonomy'=> 'category');
$categories =  get_categories($args);

if ($categories){
                        foreach ( $categories as $category ){ 
                            if(in_array($category->term_id,(array)$data)) {
                                $selected = 'checked="checked""';
                            } else {
                                $selected = '';
                            }
                            echo '<input name="user_cats[]" value="'.$category->term_id.'" '.$selected.' type="checkbox"/> '.$category->name.'<br/>';
                        }
                    }

This is what the options look like in the user profile page:

After doing a test save of the some of the checkbox selections, I examined the database value of the "user_cats" field, it gives me something like this:

a:2:{i:0;s:2:"20";i:1;s:3:"343";}

Now, I know what all but one of the variables stand for.

a = Total amount (quantity) of selections

i = Sequential number of the selection (*Starting with 0)

"X" = The number value within the parenthesis is the category ID

My question is, what is the s: value based on? I tried comparing to