It's just saying that if you have used Unleash Flame (which debuffs the target for a +30% fire dmg for 8secs; only to the next fire spell), and that there is enough time left on that for you to cast lava burst without flame shock dropping off, then cast lava burst.
Depending on how much time is left on the Flame Shock dot, you may not need to immediately follow that Lava Burst with a Flame Shock.
Just to be clear ... Unleash Flame debuff is what is applied when you unleash elements with Flametongue applied to your weapon.