Setting: Matthew 13 + Mark 4 in the Galilee Kingdom-parables cluster; Luke 13:18-19 spoken later during the Peraean ministry. Citation: Matthew 13:31-32 || Mark 4:30-32 || Luke 13:18-19
The parable
The Kingdom of Heaven (Mat) / God (Mrk + Luk) is like a mustard seed, the smallest of garden seeds, but when grown becomes a tree large enough for birds of the air to nest in its branches.
The main point
Small beginnings → great consummation. The Kingdom may appear unimpressive in its initial form (a wandering rabbi + 12 disciples + outcasts) but will grow to fill the world. Read alongside the Leaven parable that follows immediately in Matthew 13 + Luke 13 (same point with different imagery).
Botanical note
The black mustard (Brassica nigra) common in 1st-c. Palestine had seeds ~1mm diameter, proverbial smallness ("if you have faith as a grain of mustard seed", Matthew 17:20; Luke 17:6). The mature plant could reach 3-4 meters tall, large by garden-plant standards but not literally tree-sized. The hyperbolic "tree" / "birds nesting in branches" language echoes OT imagery:
Ezekiel 17:22-24, God plants a sprig that becomes "a good cedar" whose branches shelter "Birds of every kind"Ezekiel 31:6, Pharaoh-as-cedar; "All the birds of the sky made their nests in its boughs"Daniel 4:10-12, Nebuchadnezzar's vision of the great tree; "the birds of the sky lived in its branches"
The Kingdom of God IS the great tree of OT imagery, but it begins from a tiny seed.
Theological framing (multiple views)
Kingdom-growth reading (mainstream), the parable predicts the Kingdom's expansion from its small beginning in Jesus's ministry to its eventual world-encompassing scope. Optimistic about gospel-spread.
Postmillennial reading, emphasizes the gradual gospel-leavening of the world; Kingdom grows organically through history until consummation. Mustard seed + Leaven parables together are pillars of postmillennial expectation.
Amillennial reading, Kingdom grows spiritually through history (church age); growth is real but mixed with opposition (tares parable balances optimism); consummation is at Christ's return.
Premillennial reading, present-age Kingdom growth is real but limited; full consummation awaits the millennial reign. Some dispensationalists read the "birds" pejoratively (cf. the birds in the Sower parable that snatch the seed, Matthew 13:4,19) as suggesting that evil mingles in the visible Kingdom; this is a minority interpretive choice.