Meaning
The kingdom of the heavens is rare, and rare or 'few' are those who enter as well: as few as mustard grains are used as seeds. The all-important Action in this metamorphosis shows what must be undertaken: the earth must be worked upon, that is the quintessential action. If done so, that same earth - and not the grain of mustard! - will put forth, outward, a great branch.
And that branch, or earth, as they both are masculine, will offer Protection from the proverbial birds of the heaven