Legends of Runeterra has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Fleeting keyword.
Fleeting cards discard from hand when the round ends.
Blink and you'll miss it; that's how quickly a Fleeting spell can fade away. In exchange they tend to be very powerful, so use 'em or lose 'em!
Fleeting is much like the spell equivalent of Ephemeral, in-that the spell will disappear at the end of the round if not used. Careful planning might be required to insure you still have enough Mana to play them, because you certainly wouldn't want to waste such a valuable opportunity.
Keywords that Work Great with Fleeting
Fleeting is a self-contained keyword, and as such doesn't interact with the other keywords.
Playing Against Fleeting in Legends of Runeterra
There really isn't anything you can do against Fleeting, as the spells (or turrets) will disappear at the end of the round on their own if not played. The only thing you can do is pressure your opponent into using their Mana and Spell Mana, thus preventing them from playing their Fleeting spells.
Example Legends of Runeterra Fleeting Cards
Ezreal creates Fleeting Mystic Shots, allowing him to constantly poke for damage against anyone and anything (including the enemy Nexus). Barrage the enemy with spells and watch them crumble beneath your fury.
Heimerdinger creates a Fleeting turret when spells are played by you, turrets whose size are equivalent to the cost of the spell played. He lends himself to a spell-heavy deck, wherein you try to influence the table with your spells whilst dropping heavy mechanical units for your opponent to deal with.
Jinx creates Super Mega Death RocketBADCARDNAMEs whenever your hand becomes empty, turning what was once a negative (an empty hand, being forced to "top-deck") into a positive (unlimited rockets). Blow up the world, starting with your hand!
Lux creates a Fleeting Final Spark whenever you cast enough spells, allowing her to burn through an enemy with the game's currently-only example of spell-Overwhelm.