# a mustard seed

A mustard seed is a very small seed that grows into a large plant. Alternate translation: "a small seed"

# threw into his garden

"planted in his garden." People planted some kinds of seeds by throwing them so that they scattered in the garden. 

# a big tree

"a very large shrub"

# birds of heaven

"birds of the sky" or "birds that fly in the sky" or "birds"