The word "sacred" here - taken from the Greek word ἅγιος (hagios) meaning "most holy thing" according to the lexicon - refers to the confidential teachings relating to the spiritual realm and the Supreme Being.
These were the primary elements of Jesus' most valuable teachings - which were coming from the Supreme Being, as Jesus stated elsewhere:
“My teaching is not my own. It comes from the One who sent me." (John 7:16)
In other words, Jesus was guarding the confidential teachings coming from the Supreme Being.
And because Jesus was giving this instruction to his disciples, Jesus' point relates also to this confidential nature of his teachings, as well as the teachings of the prophets before him:
Yes, these teachings have been carefully guarded over the centuries.
Jesus confirmed the confidential - secret - nature of these teachings elsewhere:
"The secret of the kingdom of God has been given to you. But to those on the outside everything is said in parables...." (Mark 4:11-12)
Jesus sometimes spoke in parables so that only those who were humble enough to be able to receive spiritual understanding could receive the message. Others -- those with their own agendas -- were left puzzled about the message of the particular message.
God communicates His lessons and truths through His confidential servants and representatives. From Abraham to Moses to David to John the Baptist and many in between and from other cultures, lineages and times we find the same occurrence. We find the Truth passed down by God's prophets being bent and misdirected by those who have attempted to utilize them for self-centered purposes - whether personally or institutionally.
In other words, we find even the available Truth has been neglected or criticized over the centuries by those who were not humble and receptive enough to hear the Truth as it is.
These are the "pigs" that Jesus is warning us about.
For this reason, Jesus was teaching his disciples to carefully present the Truth to those who are ready to hear it. He wanted them to carefully reel out the right amount of wisdom to meet the situation and the depth of the listener, and avoid the Truth being trampled upon and misused.
In terms of students of Jesus' teachings, there is another message. That is, the teachings of God's representative must be received with humility and reverence.
This point was confirmed by another statement by Jesus:
“I praise you, Father, Lord of heaven and earth, because you have hidden these things from the wise and learned, and revealed them to little children." (Matt. 11:25)
(For a translation of Jesus' statements from the Gospel of Matthew without institutional sectarian influence, see the Devotional Translation - translated from the original Greek texts.)