What is the rhythm of the poem "Because I could not stop for Death"?

The poem's written in ballad meter, alternating lines of iambic tetrameter and iambic trimeter, rhyming ABAB. I'll show you what I mean.

An iamb is a weak beat followed by a strong beat (de DUM). So all you have to do is count the iambs:

de DUM de DUM de  DUM  de  DUM    (4)
Because I could not stop for Death,
de DUMde DUM    de DUM                 (3)
He kindly stopped for me;
de  DUM de  DUM de  DUM de DUM     (4)
The carriage held but just ourselves
de DUMdeDUMdeDUM                        (3)
And Immortality.

A line with four iambs is an iambic tetrameter. A line with three is an iambic trimeter. And this form is commonly thought of as "ballad meter". Hope it helps!