Suppose you are playing a hand of Euchre, and the dealer is to your right. Now imagine that your partner, across from you, has called trump. Maybe they ordered up the up card into the dealer's hand. Or, maybe they called trump after everyone else had passed. Since you are left of the dealer, you must lead the first trick. What card should you play? If you hold a trump card, you should lead the trick with that card.
Here's a replay of a Euchre hand that shows you why. You can also watch how it works in the YouTube video below.
In this example, I am right of the dealer. Everyone has passed. I have two black jacks: the jack of clubs, and the jack of spades. I also have the queen of clubs. If I call clubs as trump, I will have three trump cards, and the ace of diamonds. That's a pretty strong hand. So I call clubs.
My partner must play the first card. What should they do? As you can see, my partner has a strong hand, also: two club (trump) cards, and the ace of hearts. It might be tempting for them to lead the trick with the ace. It seems like a sure winner. But it's not. If you look at our hands in the replayer, you'll see that my right-hand opponent is void in hearts. That means that if my partner plays a heart, the opponent does not have to follow suit. They can trump this trick! Because I do have a hearts card, I will have to follow suit. So my partner's ace, which might have taken a trick, is lost.
Fortunately for my team, my teammate recognized that since I called clubs as the trump suit, I probably have a strong hand with several clubs, and very probably have the right bower - the jack of clubs - which is the strongest trump card. Otherwise, as "third chair" (another name for "left of dealer"), I would have passed on calling trump, and forced the choice of trump onto the dealer. With this logic in mind, my teammate elects to lead with a low trump card. This move will strip trump from the hands of our opponents. In fact, it is likely that the opponents will not have any trump cards after this play. Then, as you see, any aces that we have can be played without fear that they will be trumped.