This is the 2352nd most frequent Spanish word.
testigo
Witness.
In this case, 'testigo' is used metaphorically as a noun denoting evidence or proof for an abstract concept, showing the figurative extension of its meaning.
Su confianza en él es un testigo de su fuerte amistad.
Her confidence in him is testimony to their strong friendship.
In this sentence, 'testigo' is used as a noun meaning 'witness' to indicate someone who saw or experienced events personally.
Ella fue testigo de los eventos aquella noche.
She was a witness to the events that night.
Here, 'testigo' is used again as a noun referring specifically to a legal witness who presents evidence in a court of law.