What is class=“mb-0” in Bootstrap 4?

It is used to create a bottom margin of 0 (margin-bottom:0).

Bootstrap 4 has a wide range of responsive margin and padding utility classes. They work for all breakpoints: xs (<=576px), sm(>=576px), md (>=768px), lg (>=992px) or xl (>=1200px)):

The classes are used in the format: {property}{sides}-{size} for xs and {property}{sides}-{breakpoint}-{size} for sm, md, lg, and xl.

Where property is one of:

• m - sets margin
• p - sets padding

Where sides is one of:

• t - sets margin-top or padding-top
• b - sets margin-bottom or padding-bottom
• l - sets margin-left or padding-left
• r - sets margin-right or padding-right
• x - sets both padding-left and padding-right or margin-left and margin-right
• y - sets both padding-top and padding-bottom or margin-top and margin-bottom
• blank - sets a margin or padding on all 4 sides of the element

Where size is one of:

• 0 - sets margin or padding to 0
• 1 - sets margin or padding to .25rem (4px if font-size is 16px)
• 2 - sets margin or padding to .5rem (8px if font-size is 16px)
• 3 - sets margin or padding to 1rem (16px if font-size is 16px)
• 4 - sets margin or padding to 1.5rem (24px if font-size is 16px)
• 5 - sets margin or padding to 3rem (48px if font-size is 16px)
• auto - sets margin to auto

Read more in w3schools