From type/Match

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method make(Match:D: Mu $payload)
sub make(Mu $payload)

Sets the .ast attribute, which will be retrieved using .made.

$/.make("your payload here");

That is, it stores an arbitrary payload into the Match object that can later be retrieved via .made method. Since the sub form operates, by default, on $/, that example is equivalent to:

make("your payload here");

This is typically used in a grammar's actions class methods, where a piece of data is stored by one method and then later retrieved by another. It's up to you what data you store. It could be a tree node, result of a calculation, a type object, or a list of values.

The sub form operates on the current Match $/, which can be a convenient shortcut:

method my-action ($/) {
    make "foo: $/";
}