I know bier has something to do with dead people, but what is it exactly?

A bier is a platform or portable framework on which a coffin or corpse is placed. It's also the coffin and its supporting platform.

In Book VI of Virgil's Aeneid, Aeneas discovers the dead body of his trumpeter Misenus, who was drowned by the sea god Triton for daring to challenge him in a trumpeting contest. Aeneas and his companions give their fallen comrade the due rites of cremation and burial:

Loud is the wailing; then with many a tear
They lay him on the bed, and o'er him throw
His purple robes. These lift the massive bier;
Those, as of yore — sad ministry of woe —
With eyes averted, hold the torch below.