Tier 1: 5/5 Rating
Pokémon in Tier 1 are meta-defining. In no uncertain terms, these are the threats to look out for. If you’re not ready to beat these Pokémon, you’re probably going to lose to them. They are typically the best at filling whatever role they fit into, whatever that may be. Their power level is so high that they make their role crucial to the metagame of the League as a whole.
Tier 2: 4 to 4.5 Rating
Tier 2 Pokémon are still very strong. They may not be meta-defining, or they might get pushed out of their niche by a Tier 1 Pokémon, but that doesn't change the fact that they're incredibly threatening in their own right, and are likely to be top picks for your team.
Tier 3: 3 to 3.5 Rating
Tier 3 Pokémon are likely to be very useful in limited metagames (such as Silph Arena Cups), and most of them are still very reasonable picks in open formats. Just keep an eye out for higher tier picks that might fill the role better.
Tier 4: 2 to 2.5 Rating
If a Tier 4 Pokémon is legal in a Silph cup, it’ll probably see play. But if someone brings it in an open format tournament, they’re probably up to something. There are typically going to be multiple Pokémon that will fill a T4 mon’s niche better.
Tier 5: 1 to 1.5 Rating
They...might be useful? Sometimes? With niche cup relevance at best, these Pokémon are probably going to be on the bench more often than not.
Not on the Tier List: 0 Rating
Pokémon rated 0/5, while they may not technically be completely useless, should definitely be avoided. You can use them if you want to show off how cool you are by not following the meta, but you’re better off looking elsewhere if you like winning.
What Do I Do with This Information?
As nice as it would be, you can’t just arbitrarily choose 6 Tier 1 Pokémon and expect to be victorious. While, on average, a Tier 1 Pokémon will be stronger than one in Tier 3, that doesn’t mean the Tier 3 one won’t fill a role in your team better.
That said, do not just look at the tier of each Pokémon. Like with any tier list, each entry has a significant amount of nuance to it, so your best bet is to actually look at the writeup for each Pokémon to get a better sense of how they work!