This little imperative phrase is the most frequently occurring command that Jesus gave. You will not find any command that is more often used than this. It usually follows some principle that Jesus has been teaching, and He used it to encourage the audience to pay attention.
Paying attention to what God has communicated is the smartest thing you can do. The central contention of Christianity is that God communicated information vital for our well-being. This includes the good news that Jesus paid the penalty for our sins and that God accepts this payment. It also includes the good news that Jesus is coming back to set up a kingdom in which He will reward His faithful servants. And it includes many principles and instructions that will increase the quality of our current life. John said that Jesus is the Word of God and came so that we may have life, and have it more abundantly. So when God speaks, we better pay attention, because it is good for us.
The phrase is structured as a conditional clause. Only those people should hear who have ears to hear. There is some healthy sarcasm going on here. Everybody who was part of the audience had ears. They were already listening to the parables and teachings of Jesus. And the purpose for which we have ears is not so that we don’t need sails on our sailboats, but so that we can hear.
But there is something deeper implied by this little phrase. By encouraging people to listen, He acknowledges that not everyone who has ears will actually use them to listen to Him. This points to another important theme of Christianity: free will. God has communicated information that is truly vital for our well-being, and some people decide to listen to Him, and some decide to ignore Him. God does not force a relationship with Himself on us. He told us what we need to know, and then invites, or calls, us to respond to Him. But the choice is ours. We are free to choose to listen, and we are free to choose to ignore. But we are not free from the consequences of our choices. God is the sovereign ruler of the universe; He created it, so He sets the rules. But He does not force us to play by His rules, He does not micro-manage our lives. But He will keep His promises and there are unavoidable consequences attached to our choices.
An infinitely loving God loves us so much that He gives us free will. An infinitely just God will attach consequences to our actions; good to good, and bad to bad. And an infinitely fair God will communicate these consequences clearly and audibly.
God has spoken. Are you listening?