As with everything that may not apparent immediately- we need to read the complete text in its context.

Matt 23:1-12: “Then Jesus spoke to the multitudes and to His disciples, v.2 saying: "The scribes and the Pharisees sit in Moses' seat. "Therefore whatever they tell you to observe, that observe and do, but do not do according to their works; for they say, and do not do. "For they bind heavy burdens, hard to bear, and lay them on men's shoulders; but they themselves will not move them with one of their fingers. "But all their works they do to be seen by men. They make their phylacteries broad and enlarge the borders of their garments. "They love the best places at feasts, the best seats in the synagogues, "greetings in the marketplaces, and to be called by men, 'Rabbi, Rabbi.' "But you, do not be called 'Rabbi'; for One is your Teacher, the Christ, and you are all brethren. "Do not call anyone on earth your father; for One is your Father, He who is in heaven. "And do not be called teachers; for One is your Teacher, the Christ. "But he who is greatest among you shall be your servant. "And whoever exalts himself will be humbled, and he who humbles himself will be exalted.”

After this is said Jesus goes into 7 woes to the Pharisees that were also present.

The Old Testament may not say these exact words “Moses' seat,” but there are lots of terms and phrases that are interpreted in the New Testament that have its roots and meaning in the Old Testament. The phrase has the idea of wanting to be the leaders of Israel as Moses was.  The Pharisees were public teachers that had the office of explaining that Law and it was the custom to sit while they expounded the law and prophets (Matthew 5:1; Luke 4:20-22).

By the seat of Moses, it meant that they had the authority to teach the law. And Jesus affirms that they should be listen to as they read Moses’ law for it was the Scripture. But then He tells the people not to act the way they do “for they say, and do not do.” They went beyond the law (called the tradition of the elders (Mk.7 where they made void the commandments of God by their own laws which became the tradition they obeyed.)

In other words do as they say (as long as it is Scripture) but don’t do what they do (living in their tradition).

Had they taught what Moses taught and lived it, Jesus would have had no rebuke. Jesus then points out how they want the best of everything and be noticed because of their teaching position, he then explains they are to be servants, not self exalting themselves and that He instead of they should be their teacher.