Certain people hold power over others.
Someone else has power over you when they can deny you something that you want or need, such as affection, money, or shelter. In this way parents hold power over their children, employers hold power over their employees, and landlords hold power over their tenants.
Power is usually embedded in some kind of relationship where each party of the relationship desires something from the other. A parent and child both want affection from each other. Spouses typically want both affection and financial assistance from each other. An employer wants control over an employee’s time and behavior in exchange for providing money to the employee.
A power imbalance occurs in a relationship when one party has noticeably more power over the other party by virtue of being able to deny something significantly more valuable than the other party can deny. In considering parents and young children, a parent has greater power because they provide not only affection but also shelter, food, and money to the child, whereas the child only provides affection back. In a relationship between a working spouse and a non-working spouse, the working spouse has greater power because they are providing financial support in addition to affection.
Relationships can be terminated. This is the ultimate exercise in power, since it implies permanent withholding of exchange between both parties. Boyfriends and girlfriends can break up. Spouses can get divorced. Family members can disown each other. Employees can quit or be fired. Citizens can emigrate from their country to a different country.
If there is a power imbalance in a relationship then the less-powerful party is at increased risk of mistreatment from the more-powerful party because the less-powerful party has more to lose from terminating the relationship than the more-powerful party does. I speculate that spousal abuse, for example, occurs more frequently in the case where the abuser is the economic provider in the relationship.
Since a power imbalance in a relationship exposes one to mistreatment, it is desirable to pursue relationships where one has similar power to their partner. Unbalanced relationships are certainly possible but require a greater degree of trust.
For individuals that exhibit an unusually high or unusually low degree of personal power relative to their peers it can be hard to form balanced relationships. More powerful individuals must rely increasingly on establishing trust when forming new relationships. Less powerful individuals must be more wary of those who they are considering forming a relationship with.
The accumulation of money in particular is a proxy for power, since whoever has money can use it to influence others to action.
Historically men have on average had more power then women in the United States, particularly economic power. This explains why it is easier to visualize a husband mistreating a wife rather than the other way around. It also explains certain customs such as the man buying the drink for the woman at the bar or the man paying for dinner rather than the other way around.
In recent years women have increasingly entered the workforce and so on average have more economic power than in years past. Thus society is trending toward greater economic equality between the sexes. Still there remains a gap between the overall economic power of women versus men, in favor of the men.