M: Margin
p: Padding
t: top
b: bottom
l: left
r: right
x: x축(left, right)
y: y축(top, bottom)
1: .25rem(if font-size is 16px then it's 4px)
2: 5rem (if font-size is 16px, then it's 8px)
3: 1rem(if font-size is 16px, then it's 16px)
4: 3rem(if font-size is 16px, thn it's 48px)
md: medium
g: grid
so, mb-3 means that it's margin-bottom that has 1rem.
