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 Tough keyword.
Takes 1 less damage from all sources.
Tough is an interesting keyword that makes units harder to kill because they take a bit less damage. Tough is almost exclusively found in Demacia.
Keywords that Work Great with Tough
Challenger is a good keyword to combine with Tough where you can now pick and choose what you kill and survive.
Playing Against Tough in Legends of Runeterra
Playing against Tough can sometimes be a bit... tough.
When in combat, you essentially need to think of a Tough unit as having +1 Health. Against a 1-Attack unit, a Tough unit will basically get free damage or a kill on it. Don't use a 1-Attack unit to block a Tough unit unless it prevents significant Nexus damage. It will also resist damage spells, do again, think of a Tough unit as having +1 Health in terms of damage. Destroy effects however will destroy it outright.
Example Legends of Runeterra Tough Cards
Plucky Poro is a basic representation of the mechanic as a 1/1 with Tough. You've got to essentially think of it as a 1/2 for combat purposes to get past it. That and it's also a Poro, and Poros are adorable.
Chain Vest is the only card in the game which can grant a unit Tough. This can be useful to drop on an intimidating unit, but it's especially useful for Fiora strategies.
Radiant Guardian is a unit that will grant itself Tough and Lifesteal if an ally already died that turn as soon as you play it. When granted Tough this way, it becomes a 5/5 with Tough and Lifesteal for 5 which is pretty impressive.
Tianna Crownguard doesn't need Tough to be an intimidating card, but it has it anyway just to make it that much worse to deal with.