“A true witness delivers souls, but a deceitful witness speaks lies.”
The word ‘souls’ in this verse is another way of saying ‘lives’. The witness is the person testifying before some judicial authority. It is important that person speaks truth so that the innocent will go free and the guilty will be convicted. The contrasting element in the proverb is a deceitful witness. Such people speak lies and pervert the cause of justice. Earlier in Proverbs 14, the same principle is communicated:
“A faithful witness does not lie, but a false witness will utter lies.”
Some might explain the verse in the context of soul winning. This can be very dangerous at times and innocuous at other times. Regretfully, I have done this myself. But it is dangerous. We must rightly divide the Word of God and avoid assuming apprehension of a passage. Application might justifiably broaden to soul winning in this text, but I would begin with what the text actually means.