The letter mem represents the revealed and the hidden: Moses and the Messiah
Mem is believed to derive its shape from the egyptian hieroglyphic symbol for water, which had been simplified by the phoenicians and named after their word for water, mem.
The history of the jewish people is characterized by exile and redemption. The first exile was in mitzraym, Egypt. The initial open mem says that at first Egypt was open and that jews were free to move in and out of the country and their homes. The final mem, a closed one, symbolizes that in the end the country was closed to them and they found themselves enslaved and trapped with no way out. (source: Sisfei kohen)
In the Sefer Yetzirah, the letter mem is King over Water, Formed Earth in the Universe, Cold in the Year, and the Belly in the Soul.
Mem corresponds to the number 40 and represents the time necessary for the ripening process that leads to fruition:
40 days for the embryo
40 years in the desert before reaching the holy land
40 years development before Moses was prepared to be the leader of Israel.
Below you can see the movie of the production process of a clay prototype of a hebrew letter.