"Nursing your husband" is a term that means feeding a husband breast milk, similar to how a mother would nurse her child. Often this is seen as a fetish, but it can also be done out of curiosity or done to clear a blockage caused by milk in the breast.
Feeding breast milk to one's husband is mostly a niche fetish. Breasts are usually seen as sexual, so some may see breast feeding as a sexual act as well. Certain individuals may perform this act out of curiosity, and sometimes it may be done accidentally during a sexual act after giving birth. Sometimes breast milk creates a blockage in the woman's breast that needs to be removed and the child or the breast pump is unable to remove it.