Question

What are they? I never understood what they were. I want a good and detailed answer.

Answer 1

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.

Source

Comments

Thanks to all of you.

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you are not welcome you nub xP
i kid, ofc c:

Answer 2

101 Subs mean that they are Subs that have the equivalent of 101 HP. Basically, any Pokemon with a Base 100 HP will have a 101 HP Sub.
Also very nice vs Chansey/Blissey, who rely on Seismic toss and such.

Answer 3

They are for moves like seismic toss and night shade that will always do 100 HP. People like to have 404 HP Pokemon so that 1/4 will go to the substitute. But overall it is to keep your substitute after a blissey, or chansey or whatever uses seismic toss or a jellicent, or dusclops or whatever uses night shade.
Source: experience