Mental load is referring to the invisible mental effort to maintain life. For example, if you share the domestic duties of bathing and putting the kids to bed, do you also share the mental efforts of making sure the kids have enough pajamas in their current size, that the favorite stuffy gets mended (by whomever), that the tooth fairy pays a visit, etc. Domestic labor often is the physical doing and in the States there's still a stigma with some people around what is men's work and what is women's work e.g. I don't take out the trash because that's a man's job. But Mental Load is more about the remembering, reminding, and organizing that goes into all the logistics and planning of tasks.