A testimony is simply declaring to others what God has done for us. In Revelation 12, we see that the word of our testimony is powerful. We read that the enemy can be defeated by the blood of the Lamb and by the word of our testimony.

“Now have come the salvation and the power
    and the kingdom of our God,
    and the authority of his Messiah.
For the accuser of our brothers and sisters,
    who accuses them before our God day and night,
    has been hurled down.
11 They triumphed over him
    by the blood of the Lamb
    and by the word of their testimony;
they did not love their lives so much
    as to shrink from death.
12 Therefore rejoice, you heavens
    and you who dwell in them!
But woe to the earth and the sea,
    because the devil has gone down to you!
He is filled with fury,
    because he knows that his time is short.” (Rev 12:10-12)

As we share testimonies, we are effectively declaring that God is great, that God does great things, that God loves us and cares for us. As we share a word of testimony, we remind others that God is a personal God, that God knows each one of us by name, that the very hairs on our head are numbered, that we are never forsaken. The word of our testimony is powerful because its focus is not on ourselves but on God. Many of the psalms act as people’s testimonies. In Ps 118, David says, ‘I was pushed back and about to fall, but the Lord helped me. The Lord is my strength and my defence; he has become my salvation.’ (Ps 118:13-14) David was in a difficult position. David was facing enemies on all sides and felt hemmed in; he felt oppressed and as though he were surrounded by swarming bees. (Ps 118:12) But God stepped in. God helped. He goes on to say,

‘Shouts of joy and victory
    resound in the tents of the righteous:
“The Lord’s right hand has done mighty things!
  The Lord’s right hand is lifted high;
    the Lord’s right hand has done mighty things!”’ (Ps 118:15-16)

Testimonies remind us that God has done mighty things and continues to work on our behalf.