Modifier Tiers
Modifiers have a tiering system, which affects how strong a modifier is. Think of tiers almost like a “level”: a low tier Life mod might only grant between 10-20 to Life, but a high tier Life mod could grant 190-200 Life. The value of the mod cannot be changed except within its value range -- this is important to know because you might find an item with all the right modifiers, but with low mod tiers and it would be wasteful to use an expensive Currency item like a Divine Orb to try to get better rolls above the maximum range of that mod tier.
Modifier tiers in Path of Exile 2 are different from Path of Exile 1 in that the lowest tier is now Tier 1 and the highest tier varies based on the modifier and item on which it appears. For example, +# to Maximum Life on Body Armour has 13 tiers, with Tier 13 being the highest and tier 1 being the lowest. However, Spirit only has 8 tiers, with Tier 1 being the lowest and Tier 8 being the highest.
To further complicate things: a Helmet has only 10 tiers of +# to Maximum Life, with Tier 1 being the lowest and Tier 10 being the highest
Item Level
The tier of modifiers that can appear on an item is affected by the Item Level of that item. Items with a higher item level can roll mods with higher mod tiers. Item level is affected by the area level in which you find that item, meaning that you will find items with higher item level the further you progress in the game.
Items with low Item levels can't "get lucky" and somehow roll a Tier 7 modifier by chance: these higher item tiers are reserved for higher item level items.
Don't Panic
It isn't expected that you work all of this into memory immediately. Once you expand the mod pool to reveal all tiers of modifiers, you'll see that there are thousands of mods in Path of Exile 2, and learning all of this isn't easy or even all that useful.
What's much more useful is to start learning various breakpoints of important modifiers. As an example, the highest tier of Elemental Resistance is +41-45% and starts appearing on items with an Item Level of 82 and above. When the word "Elemental Resistance" is used in this case, it implies Fire, Cold and Lightning Resistances, but not Chaos Resistance. Chaos Resistance has a maximum tier that can be found on Body Armours with an item level of 81 or higher, instead of 82 as is the case with Elemental Resistances.
On Boots, the highest tier of % Increased Movement Speed requires Item Level 82 to appear. For the second highest Tier, Item Level 70 is required. The type of Boots you find won't affect how likely a specific tier of Increased Movement Speed will appear and this remains static across all Boot types.
Learning these breakpoints will take time, but it is a much more practical way to start familiarising yourself with the mod pool.
How to See Prefix/Suffix/Mod Tier/Item Level
You're probably wondering "But how do I see if a modifier is a prefix or suffix? How do I know its tier? How do I know what item level my item even is??"
Good news: you can see this all in Path of Exile 2. By pressing Alt while hovering over an item, small grey text will appear to the right and level of the modifiers on your item. This is very easy to miss but in the image below, these sections are highlighted inside the yellow box
To the left, you will see Implicit, Prefix and Suffix brackets which let you know which mods are Prefixes and which are Suffixes.
To the right, you will see the Tiers listed as "T7, T7, T7, T2..." etc. "T" is a shorthand way of writing "Tier"
Unfortunately, as of the time of writing, it isn't possible to see a list of the full mod pool in game to start learning the above-mentioned useful breakpoints and third party resources will need to be used to see all of those. It isn't extremely important to know these exact breakpoints until much later on into the endgame when tackling more ambitious crafts, so don't feel scared to craft on lower level items that you find to create possible upgrades.