What does " .align " do in MIPS?

I'm taking this course and been confused by this instruction. Please tell me what it follows with, like when should it be followed by 4 and when by 8.

e.g.

.data

.align 8

STR1A:

.asciiz "Enter a string: "

why here is 8

e.g.2

int A [100] , B [100];

transferred into C code will be

.data

.align 4 %integer-aligned

A:.space 400 % first array

B: .space 400 % second array

why here is 4