Hi Liz, 

The verb 'taire' (not to say/ to keep quiet about something) and which past participle is 'tu' is used very rarely in this way, but you might encounter it as a reflexive 'se taire' -

Taisez-vous! Tais-toi! = Keep quiet/ shut up!

Il s'est tu devant elle = He kept quiet in front of her

In the lesson example -

Il a tu les raisons de son départ, it means,  he didn't give the reasons for his leaving 

The word 'about' is not necessary in French but I hope this explanation is helpful!