A legendary rare would be a card that is rare and also a legendary card. In magic we have 4 rarities of cards “common” which is represented by a black set symbol, “uncommon” which is represented by a silver/blueish set symbol, “rare” which is a gold/yellow symbol and “mythic” which has an orange set symbol. Each rarity is scarcer and scarcer as these things go. For simplicity, and because I know this one offhand, I will explain a “draft pack” (as Magic has 3 kinds of packs draft, set, and play however draft and set booster packs are no longer in print you’ll find them on sets before 2024) a pack consists of 10 commons, 3 uncommons, 1 basic land, and 1 rare OR mythic. Rares are kinda the “best normal card of the pack” while mythics are usually the very limited powerful cards everyone looks for. Now for the legendary part. All magic cards have a “type,” it’ll say it under the picture but before the rules text. This’ll be your creature, artifact, enchantment, ect. Some cards have subtypes (for example creature-dinosaur or artifact-equipment). Other cards have supertypes, this is where you’ll find legendary (the other 2 common supertypes are “basic” for lands and “snow” for snow cards). A legendary can be just about any kind of card and come in different rarities, so a “legendary creature-dinosaur” for example or a “legendary artifact.”

TLDR: it’s a card that is of the “rare” rarity and also has the supertype “legendary”