The Legendary card from Jade for example, is Aya Blackpaw.
As you can see from the card text you summon a Jade Golem from both the Battlecry and the Deathrattle of this minion.
Jade Golems are minions that start at 1/1 when you summon the first one. They then grow +1/+1 in stats for every extra Jade Golem you summon.
So, Aya Blackpaw will summon a 1/1 when you play her. Then when she is killed off you will see a 2/2 spawn. (unless you have summoned more in the meanwhile).
It’s not clear yet how many cards will summon Jade Golems in a single game. The 30/30 is possible, but just used here as an example.
With Brann Bronzebeard doubling up the battlecries there is some potential for a lot of Golems hitting the board. We will have to see how it plays out when the new expansion is released in a few weeks.