Pilate asked the question "What is truth?" A poignant statement oft repeated today. We live in an age when many choose to answer this by saying its no more than an opinion By contrast Jesus states that He is the only personification of The Truth.
In doing so Jesus is claiming that there is only one Truth. Linking it with the fact that He is also the only Way that ensures eternal Life. Any other viewpoint, no matter if it seems credible,can achieve this, for No one comes to the Father unless they accept , this statement to be the Truth.
When we surrender our lives in repentance and faith to Almighty God The Spirit Himself then dwells within us, His primary purpose to lead us into all Truth. The answer to this question therefore is that Truth is the reality of the triune God. The Spirit also inspiring the writing of the Scriptures known as The Word of God
So it is as we read and learn of the gospel that we are led into all truth as we have need. Our task then being to share with others the good news of salvation from the power of sin. That in repentance and faith in the Lord Jesus Christ they too may know the Truth, and that, as we read in John's gospel we will then 'be free indeed'.