Critical Rate is the chance for your attacks to do additional damage. For example, you have a 20% crit rate, there is a 1 in 5 chance that your attack will do more damage.
Now, how is this additional damage computed? That depends on your Critical Damage. A 1k attack with 20% crit damage will do 1.2k damage.
Iframes are skills where your character will become untargetable by attacks. You will know this when a big red X mark is shown when your character uses a skill.
DPS means damage per second. Self explanatory.
Critical rate is how often you will do critical damage. Not as important as ignore defense, skill cooldown, and critical damage. Iframes is a skill where you cant take damage during the whole skill animation, Quicksilver is a prime example. DPS is damage per second, basically how powerful your character is in terms of damage/strength. Scarlet Witch, Deadpool, and Cable are high DPS in my opinion although there are others :)