Three things govern your ability to operate in the environment — hit points, mana level, and movement points. If you’re new to this kind of game, these three things need some explanation. Hits are a measure of health. Mana is a measure of your spell casting ability. Movement is a limitation on how far you can walk before resting.
Hit points, or hits, are your critical factor. If they go into negative numbers, you’re unconscious. If they go far enough down, you die. You can re-enter the game — but naked and broke, and barely alive. Your body and all it held is whereever you died. Hit points are a function of your character’s constitution and your level. The more levels you have, especially in your primary, the higher your hitpoints. Racial modifiers have an effect on constitution — ogres have most — and that translates directly into hitpoints. A “big” warrior might have 500 hps.
Mana is magic power and used by both clerics and mages. Each spell you cast uses mana points up. The number of points depends on the level of spell and the level of the caster. A level one spell like “magic missile” can be cast by a level one mage, but it will take a lot of power. The same spell takes less power as you move up the ladder. A level 10 mage can cast “magic missile” with a fraction of the power consumed. Of course, no magic user can cast a spell they haven’t learned, regardless of level.
Moves are a function of your load. On flat, level, smooth terrain you don’t use many. Climbing cliffs, you use a lot. The more you carry, the lower your maximum. So, you may have a maximum movement point level at 100, but pick up a chainmail and the extra weight drops you down — maybe to 50. There’s a grace period between the time you pick up something heavy and the “start of the tick” so if you have to move a load, time it carefully.
For all these factors, there are objects in the game that can help you. Bracers that give you +10 hitpoints, and boots that give you +20 moves. You might wield a weapon that adds 10 mana to your maximum. These are the most valued items in the game and the larger the effect, the rarer and more valuable they become. Also more brittle so they wear out and crumble faster.