After the starter Kit, a lot of minor things were ironed out...
What we're missing here is: one is a Defend Action (one of the basic action types), and the other is a Reaction that allows you to defend.
The big distinction to make here is between action (something you do on your turn) and reaction (something you can do anytime, once, between your turns)
In both cases it means to prep for incoming damage, and mitigate that damage by the result of your roll.
In a defend *Action*, you collect your pool, roll 3 dice and set up a 'damage sponge' for the next incoming, which is determined by your MID (usually). This is a pre-emptive action, and goes to waste if you are not attacked before your next turn.
In a defending *REaction*, you do the same, but more hastily, and roll only one die to determine the effectiveness. Thus, results are more random, and it usually appears with a trigger like "when you would take damage', so the defense will be used up immediately.
Rule of thumb: On your own turn, (i. e. when you narrate the action), use dice pools. On other's turns, a single roll must suffice, because right now it's not really about you.
Hope this helps!