As you accumulate XP, the current amount gained shows at the top center of your screen.
The PMMO button in the inventory provides an alternate way to view your skills as well as the XP needed for the next level. The button itself can be moved in the client config if it overlaps with other mods.
Most details for items can be seen in the tooltip. The client config lets you disable values you don't want to see.
If you don't meet the requirements to perform an action, PMMO will let you know, such as this example requiring 30 levels of combat to enter The Nether.
This block has a requirement to break it.
The smithing table is the default salvage block, but that is configurable. When looking at the salvage block , this tooltip will appear.
Salvage requires the player crouch. If you are also holding a salvageable item, the tooltip will change to show you the salvage details for that item. Right click the salvage block to attempt the salvage.
When a block is marked for vein mining, all blocks that will be broken highlight.
The full glossary shows many things such as how much vein mining each item in the game gives you.
When looking at a block or entity, pressing the "P" key will open the glossary for that object.
Pressing "Open Glossary" from the P menu opens this screen to let you view all settings from the mod.
In the full glossary, you can see detailed information such as the xp gained from breaking blocks.
If you don't meet the requirements to enter a biome, you will receive negative effects, such as this cold ocean requiring 25 swimming and 15 endurance or else you get weakness and slowness.