'Untruthful Testimony' Whilst Fasting

Question:

What is the meaning of 'untruthful testimony' and does it invalidate fasting?
Answer:

Untruthful testimony is one of the greatest of the major sins, and it is for a man to testify to something which he does not know, or to testify to something when he knows that it is not true. It does not invalidate the fast, but it decreases its reward.
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Arkaanul-Islaam, Vol.2, p.668