en_tn_condensed/num/15/20.md

the first of your dough

Possible meanings are that this refers to 1) the first grain that they would gather during the harvest or 2) the dough that they would make from the first of their grain. (See: rc://en/ta/man/jit/figs-metonymy)

a loaf

Calling it a loaf implies that they would cooked the dough first.

to lift it up as a contribution

This idiom "lift it up" refers to offering it as a gift. Alternate translation: "to offer it as a gift" (See: rc://en/ta/man/jit/figs-idiom)

a contribution from the threshing floor

The offering is spoken of as being from the threshing floor because this is where they would separate the grain from the other parts of the plant.