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 individual 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;
}