Placeholders are extended versions of tabstops. Instead empty space, you can defined predefined content in place of placeholder, that is going to be selected (thus easily rewritable) when moving across inserted snippet using TAB. Common notation is ${N:default value}, where N stands for tabstop number and default value is some predefined text.

class ${1:ClassName}
        def ${2:method_name}
                $0
        end
end

Placeholders can be nested, best example of nested placeholders can be seen as:

<div${1: id="${2:some_id}"}>
        $0
</div>