“leaven.” “Leaven” was something that made dough rise, “leaven” is typically yeast. Although leaven is often portrayed as an evil thing such as sin, in this parable it is a good thing, the Kingdom of Heaven. It is not uncommon for things in the Bible to be either bad or good depending on the context. For example, Satan is called a lion (1 Pet. 5:8), but so is Jesus (Rev. 5:5).
“that a woman took and hid.” No woman “hides” yeast in bread dough; they put it in so that the dough will rise. In this parable, God is compared to a woman hiding yeast in bread, and the wording reveals the purpose and activity of God. God conceals the Kingdom as people get saved and join it, and as the yeast spreads throughout the loaf, Kingdom people, saved people, spread throughout the world, mainly being ignored by the worldly people.
“50 pounds.” The Greek measure is three satons, and “saton,” was the Greek name for the Hebrew term “seah” (Sarah used three seahs of flour for her divine guests in Gen. 18:6). A saton is a little over 16 pounds (7 kg) of dry measure (or just over 13 liters). So this was over 47 lbs (21 kg) of flour total, which would feed more than 100 people. No ordinary housewife would cook that much meal, so the parable points to the Kingdom of God and His provision for all the people who would be saved and enter the Kingdom.