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/translate/figs-metonymy]])
# a loaf
Calling it a loaf implies that they would cooked the dough first.
# to raise it up as a raised offering
This idiom "raise it up" refers to offering it as a gift. AT: "to offer it as a gift" (See: [[rc://en/ta/man/translate/figs-idiom]])