PURPORT

"My Lord, You kill all the enemies of the world in Your multifarious incarnations in the families of men, animals, demigods, rsis, aquatics and so on. Thus You illuminate the worlds with transcendental knowledge. In the age of Kali, O Mahapurusa, You sometimes appear in a covered incarnation. Therefore You are known as Triyuga [one who appears in only three yugas]." (Bhag. 7.9.38)

Srila Sridhara Svami has also verified that Lord Visnu appears in the age of Kali but does not act as He does in other ages. Lord Visnu incarnates for two purposes: paritranaya sadhunam vinasaya ca duskrtam. That is, He comes to engage in pastimes with His devotees and to annihilate the demons. These purposes are visible in the Satya, Treta and Dvapara yugas, but in Kali-yuga the Lord appears disguised. He does not directly kill demons and give protection to the faithful. Because the Lord is not directly perceived in Kali-yuga but is directly known in the other three yugas, His name is Triyuga.