Smogon explains it well:
Substitute + 404 HP
The main purpose of this is to counteract Seismic Toss or Night Shade.
When a Pokémon with 404 or more HP creates a Substitute, the HP of the
Substitute has 101 HP, requiring TWO Seismic Tosses to break. This
means that if Blissey were to stay in, the Substituting Pokémon would
get a "free turn."
Tyranitar ~ Substitute ~ Crunch ~ Thunderbolt ~ Focus Punch This set,
popularly known as "Tyraniboah," was designed to counter stall teams
and specifically the Skarmory + Blissey combination. When Tyranitar
Substitutes, Blissey's Seismic Toss cannot break that Substitute,
allowing Tyranitar to Focus Punch the next turn without fear of being
hit. The basis here is a combination of both the Substitute + Focus
Punch and the Substitute + 404 HP ideas.