Within the context of feminism male allies are men that align themselves with feminism. Some feminist groups accept men as feminists but many do not. These latter groups will often have male allies. Some feminist groups do not accept any involvement from men.
Contrast with the term men's rights activist (MRA) which is gender-neutral. MRAs may be male or female. Female MRAs are sometimes called feMRAs and women who are very active within the men's rights movement are called honey badgers.
Intersectional feminism also uses the term allies more widely.