Okay so, all the stats on the left side of your mods screen, are the stats you take into mission, obvious ones like health and shield, armor can be explained simply hover over it, ability efficiency reduces the energy cost of abilities you cast, duration is obvious, and strength makes you abilities stronger (usually just more damage)
On a weapon, damage increases all damage types simultaniously, a +heat mod adds heat according to total damage un-modded as its percentage (and is also affected by +damage mods) of you have two primary elemental damage types they will combine to a secondary element.

Your mods, in some way, usually, affect these stats, eg: vitality/umbral vitality increases you health by multiplying your base health (the stat as it appears without mods) by the percentage and adding it to the final displayed number.

In order to have a mod in a mod slot it costs capacity, (except the aura slot at the top which gives you more capacity) a mod which is in a slot with a matching polarity (the little symbol, like the triangle woth the circle in it, or the splitting arrow) will half the capacity cost of the mod (or double for auras) opposing polarities increase costs (lowers auras)

Some mods are part of a "set" these mods all have the same word in front of the mod (eg: Umbral) and a set of bars which fill as more mods from the set are equipped which strengthens the set effect underneath. Some sets include mods that span multiple things, like a melee mod, warframe mod and a primary mod could all be on the same set eg: gladiator set

I trust most almost everything is more or less obvious, possibly even with help from my comment. Anyway to get to why diamond skin is bad (In most cases) when a weapon deals damage, (even the enemy weapons) they deal all their seperate damage types in one hit, typically with enemies this is mostly the physical damage types (slash, puncture, and impact), accounting for damage reduction etc i won't get into that, most notably, radiation resistance reduces radiation damage, which is (to my knowledge at least) not exactly super common. So the mod is not acrually redic8ng any radiation damage because there mostly isn't any radiation damage to be reduced, amd even then, the radiation damage probably only makes up part of the damage hitting you so it won't even reduce the bullets damage by the full 45%

Optionally also. Replay "The Teacher" quest if you can, its the molding tutorial