What is the uses of :: in CSS?

Answer 1

The double colon replaced the single-colon notation for pseudo-elements in CSS3. This was an attempt from W3C to distinguish between pseudo-classes and pseudo-elements.

example:

p::first-line {
color: red;
}

versus

p:first-line {
color: red;
}

Both segments of code will work.
For backward compatibility, the single-colon syntax is acceptable for CSS2 and CSS1 pseudo-elements.

Answer 2

For pseudo-elements Ex.: ::before and ::after