This opening verse of the Vishnu Sahasranama is a declaration of the power of remembrance (smarana) — one of the nine forms of bhakti. Just remembering Vishnu's name, even once with sincerity, can begin to loosen the knots of samsara (the cycle of birth, death, and rebirth).
The Vishnu Sahasranama is recited by millions of devotees daily, often after puja or during morning prayers. This verse sets the intention before the thousand names begin.
Prabhavishnu means one who is all-pervading and supremely potent — Vishnu is not just a deity in a distant heaven, but the very ground of all existence.