Blocks refer to reusable units within the CSS files. Where practicable, we should always try to define a general block or UX concept, rather than styling indivudual elements.
Example:
/* BAD */ /*<form id="new_user_form">...*/ #new_user_form { font-size: 1.2rem; margin-left: 14px; margin-right: 14px; border: thin solid silver; } /* BETTER */ /*<form class="bordered-form">...*/ .bordered-form { font-size: 1.2rem; margin-left: 14px; margin-right: 14px; border: thin solid silver; } /* BEST */ /*<form class="larger margined bordered">...*/ .larger { font-size: 1.2rem; } .margined { margin-left: 14px; margin-right: 14px; } .bordered { border: thin solid silver; }