Not much to explain, a mod range from level 1-15. The higher the level, the more secondary stats/ higher the primary and secondary stats.
A mod comes with 5 tiers. Each tier is represented by a colour (Highest: gold—purple—blue—green—colourless). The higher the tier, the more secondary stats it comes with (4–3–2–1–0). Tiers CAN be increased with slicing, but only MK5 mods can be sliced. (Green to blue, blue to purple etc.)
A mod ALSO has 5 (6) different rarities, from Highest: MK6-MK1. Natural ones range from MK5-MK1, represented by the glowing dots on the mod. However, you can ONLY slice MK5 mods. The higher the rarity, the more dots it has. Higher rarity means the primary stats on the mod will be higher and has a higher maximum stat. E.g a level 15 MK5 Gold health primary stat mod gives +5.88%, while a level 15 MK4 Gold health primary mod gives +4.00%. The maximum potential of the mods below MK5 are very limited. You would want to replace all of them with MK5 in the long run.
Mods belong to different sets (Health, Critical Damage, Critical Chance, Offense, Defense, Tenacity, Potency and Speed). These are indicates by the icons on the mod. By having 2 or 4 of the same mod set depending on the set requirement, they give additional set bonuses such as +15% potency or +10% speed. This is irrelevant to the tiers and rarity. Even if you have 2/4 colourless MK1 lowest potency mods at level 15, it gives you the same +15% potency.
Now that you have basic knowledge about mods, think about the character you are modding. Since most of these mods are based off a % increase rather than the occasional flat increase from secondaries, it is better if we focus on the higher original stat of the character, say a character with very low health and very high speed would benefit more from a speed set rather than a health set, since +100% of 2 = 4, while +25% of 100= 125. You want something that scales high.
Moving on to sets, remember you have a total of 6 mods on characters. That means you can have potentially 3 mod sets bonuses on a character with each mod set requiring two of its kind, or 2 mod sets with one mod set requiring two of its kind and the other set requiring 4.
This is basically what mods are about. I’ll write up a detailed guide if you are still unclear.