.d-{value} sets display to none, inline, inline-block, block, grid, inline-grid, table, table-cell, table-row, flex or inline-flex. With breakpoint infixes they show and hide content by screen width: class="d-none d-md-block" appears only from 768 pixels up, and class="d-md-none" only below. .d-print-* classes apply when the page is printed, so .d-print-none keeps navigation off paper. Elements hidden with .d-none still download their images and run their scripts; load heavy desktop-only content with JavaScript instead.