Here "mourners' food" refers to what people would eat while they mourning because they were defiled and not acceptable to God. This means Yahweh will consider the people's sacrifices to be defiled and he will not accept them. (See: [[rc://en/ta/man/jit/figs-simile]])
The unclean food is spoken of as if it were able to go places by itself. Of course, people actually had to take it with them. (See: [[rc://en/ta/man/jit/figs-personification]])