Lets break it down.
The kingdom of heaven is like to a grain of mustard seed,
which a man took, and sowed in his field:
Which indeed is the least of all seeds:
but when it is grown,
it is the greatest among herbs,
and becometh a tree,
so that the birds of the air come
and lodge in the branches thereof.
heaven = mustard seed.
mustard seed = a tiny tiny seed.
mustard seed = a herb.
least of all seeds = a tiny tiny seed.
But when a mustard seeds grows it becomes the greatest among herbs.
The Kingdom of heaven started with a few Jews getting in to the marriage, then the rest rejected him and killed the prophets, so he sent his son and they killed him also, So he sent his servants which are us Christians to go into the streets & invite everybody who would come into his Kingdom for the wedding.
You could also say that Jesus Christ was a mustard seed which was planted 2 thousands years ago. Look at the tree it has become today! It has become so big that even the terrorist in Bethlehem comes to lodge in for safety?