Beleriand was the fictional land destroyed at the end of the First Age in J.R.R. Tolkien’s legendarium.
Beleriand was a region in the west of Middle-earth during the First Age. It was the main setting for many stories later associated with The Silmarillion, including the wars between the Noldorin Elves and Morgoth, the great enemy before Sauron.
The War of Wrath brought the First Age to its conclusion. The conflict was so devastating that much of Beleriand was broken apart and submerged beneath the sea. Only a small portion of its eastern area remained as part of Middle-earth.
Beleriand is therefore not the same place as the Shire, Rohan, or Gondor, which belong mainly to later ages and stories. Its destruction helps explain why the geography of Middle-earth changes dramatically between the First Age and the settings familiar from The Hobbit and The Lord of the Rings.