Setting: Matthew 13 and 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 and 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 and 12 disciples and outcasts) but will grow to fill the world. Read alongside the Leaven parable that follows immediately in Matthew 13 and 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 and 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.