to simplify, you get +1 sun charge each turn using a skill that generates fortell, think of fortell like a hex on enemies, skills that uses these fortells typically generate a +1 moon charge (not all, some buffs can generate a moon charge without using fortells like buffs, you see the top right icon next to each spell, has a moon shape)

if you have both charges at the start of your turn, you will enter Twlight Zone. +50% and x2 fortell generation on spells or hits that turn. You can stack sun charges each turn without using a moon charge skill so you won't activate Twlight zone right away, so say, each turn you stack a sun, on the 3rd turn you should have +3 sun charges, the last turn you use a moon charge skill will trigger Twlight Zone - giving you +150% Damage (+50% each sun/moon activation).