Hebrew scholars say the letter Samekh [ס] signifies support and protection. The idea is that the perimeter of the letterform symbolizes the Creator and the interior depicts his creation, surrounded on all sides, upheld and protected by him. It’s a nice thought, and it definitely fits with the eight verses of this fifteenth stanza of Psalm 119.
We might note that while the perimeter of the samekh has substance, the interior is a void. Accurate symbolism is not always flattering.