The basics:
The goal for each die you roll is to roll equal to or under a target number.
The Target Number is determined by adding an Attribute to a Discipline, giving you a potential range of 8-17.
If the die roll is equal to or less than that Target Number, you get a success. If it is a natural 1, you get 2 successes.
The goal for your roll (called a Task) is to reach a number of successes determined by the GM. In most cases, this will be 1 (average difficulty, equivalent to TN10 in CODA). This is called the Task's Difficulty.
In most situations, you get to roll two dice (hence the 2d20 name) for a Task, meaning you get a range of 0-4 successes.
A little more detail:
If you have a Focus that applies to the Task, instead of getting 2 successes with a natural 1 on a die roll, you would get 2 successes if you roll equal to or less than the Discipline you are using for that Task.
You have one or more Traits that can be occasionally invoked to make a task easier (using your Trait in a positive way, reducing the Task Difficulty by 1), or to make a task harder (using your Trait in a negative way, increasing the Difficulty by 1). Traits are designed to be either a boon or a hindrance, depending on the circumstance. For example, the species Trait Vulcan can be used to help or hinder the character, in that logic can give a character clarity in a chaotic moment, or can be a limiting factor when the situation requires intuition to succeed.
You have Talents, that can give you a specific benefit in certain situations (either granting you an additional die to roll for the Task, or by decreasing the Difficulty of the task).
Even more detail:
If you exceed the Difficulty of the Task, you gain a plot-point-like currency called Momentum (1 for each success above the Difficulty). Momentum can be spent in a variety of ways, including adding dice to a Task roll, or gaining an Advantage (which acts like a positive use of a Trait, reducing the Difficulty of a Task by 1) or imposing a Complication (which acts like a negative use of a Trait), or gaining additional information on the subject of your Task, etc.
The GM has a counter currency, called Threat, which acts like Momentum in every way for GM-controlled characters...and the pot of Threat can be added to by players, to gain Momentum-like benefits for themselves.
Determination is a super-Momentum currency, doing the same things as Momentum, only better (like adding a die to the Task roll...but, in the case of Determination, it automatically gives you a result of 1 (2 successes)), but these can only be spent when a character invokes a personal Value (a belief or creed) or a mission Directive (mission orders, standing orders like the Prime Directive).
.....
I hope that helps a little.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz