Grandeur is an activated ability found on a small cycle of cards from Future Sight. It reads: "Discard another card named [this card]: [effect]." So to use it, you need a second copy of the exact same card in your hand, which you discard as the cost to trigger a powerful bonus effect.
The nuance people miss is that the discarded card must have the same name—not just be similar. Some of these cards reference each other oddly, but the requirement is strict name-matching. Also, this is an activated ability you can use repeatedly as long as you keep drawing duplicates, and it doesn't tap the permanent.
In Commander, Grandeur is weak because the singleton rule means you can only ever have one copy of each card. Outside of cards that fetch or copy by name, you'll rarely activate it, making these cards niche curiosities rather than staples.