What does := mean in Go?

As others have explained already, := is for both declaration, and assignment, whereas = is for assignment only.

For example, var abc int = 20 is the same as abc := 20.

It's useful when you don't want to fill up your code with type or struct declarations.

A short variable declaration uses the syntax:

ShortVarDecl = IdentifierList ":=" ExpressionList .

It is a shorthand for a regular variable declaration with initializer expressions but no types:

The := syntax is shorthand for declaring and initializing a variable, example f := "car" is the short form of var f string = "car"

The short variable declaration operator(:=) can only be used for declaring local variables. If you try to declare a global variable using the short declaration operator, you will get an error.

Refer official documentation for more details

Keep on going to page 12 of the tour!

A Tour of Go

Short variable declarations

Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.

(Outside a function, every construct begins with a keyword and the := construct is not available.)