This verse comes just after Jesus has told the parable of the sower to the crowds (Matthew 13:1-8) but before the disciples asked Jesus what it meant (Matthew 13:10).  Yet, by this short sentence Jesus was communicating a series of important truths.  Firstly, He was saying that the parable was not just a story for amusement but a teaching from God with a message to be understood.  Secondly, the meaning could only be discerned by people who had trained their hearts to understand what their ears heard.  Thirdly, some people would never understand because they would not accept the gospel.
 
The expression was also offensive to His hearers because 'ears which could not hear and eyes which were unable to see' was the Psalmist's description of idols, and idol-worshippers, in Psalm 115:2-8.  Isaiah used the expression to describe sin-hardened people who are under God's judgement - they cannot see how to be saved even if they wanted to be (Isaiah 6:10).  Jesus confirmed this in the following verses (Matthew 13:14-15).
 
As a coin has two sides, ‘judgement’ is on the other side of the coin marked 'salvation'.  Those who will not be saved are going to be judged.  Not only are idols unable to see and hear, but they prevent living people from seeing themselves in relation to God and hearing the meaning of the gospel.  In Israel, although they were devout about their religion, it was no longer the expression of God's truth (otherwise the leaders would have welcomed Jesus).  Their religion had become an idol, a counterfeit god, which could never save.
 
The same is true today.  How easily people adhere to religious rites and formulae, assuming that if they do everything right then they will be in favour with God.  They are wrong.  The gospel says that we are all wicked sinners and too deaf and blind to even know where to look for salvation.  The parable of the sower tells us that, naturally, humanity is hard and resistant to God’s Word.  But when God works on us, we start to hear and understand how to receive the seed of truth.  Then we can respond to Him and start to live fruitful and productive lives (2 Corinthians 4:3-6).