Steps and the step/action dice table are the center of nearly all Earthdawn game mechanics. Nearly all tests are based upon a combination of one of your stats and your ranks in the appropriate Talent or skill. For example, the Melee Weapons talent is based upon your Dexterity step. So, when attacking with melee weapons, you add your Melee Weapons rank to your dexterity step to get your attack step. Other things can affect your step. Once you've added all the modifiers to your test step, you then check the final step against the step chart, and roll the appropriate action dice.
Arkanabar is attacking a Theran warrior. He is using his Melee Weapons talent. His dexterity step is 8, his Melee Weapons rank is 6, and he is using the Aggressive Attack option, which gives a +3 step bonus. 8+6+3=17, so Arkanabar's Attack Step is 17. He will roll a d20 and a d10, and add the results.
One usual modifier for tests is karma. Any time karma is used for a test, you add a karma die to the action dice and spend one of your karma points, which you purchase back with Legend Points, which are Earthdawn's measure of experience.
This Theran warrior has proven very hard to hit so far. Arkanabar is a swordmaster, which means he can spend Karma on a Melee Weapons test if he wishes. He decides to spend a Karma Point on his attack. Because he is t'skrang, that adds another d6 to the roll. Arkanabar's player will now roll a d20, a d10, and a d6, and add all their results together.
Action dice are Upper Open Ended. That means, any time a die rolls its maximum value, you roll a Bonus Die of the same size and add it to the result. This means that there is a vanishingly remote chance that a weakling with a steak knife could kill a dragon in a single blow. This also means that you can not take any fight for granted--there is always a chance that somebody will get lucky enough to kill you.
The dice come up 7 on the d20, 10 on the d10, and 4 on the d6. That's 21, but since the d10 came up at its maximum value, a d10 Bonus Die is rolled. It comes up a 10 also, so another d10 Bonus Die is added to the roll. This time it comes up a 2. Arkanabar's Melee Weapons test result is 33.
If your action dice all roll 1s, then the action automatically fails, even if you're rolling five dice against a difficulty number of 2.
As a side note, in order to save space and typing, I do not separate single dice with '+' signs, only multiples of dice.