If something is consensual, all parties are in agreement that they approve of it. You and your neighbor could have a consensual agreement to share the cost of putting up a fence between your houses.
Consensual is related to the English words consensus and consent. They all go back to the Latin verb consentire, meaning "to agree" or literally, "to feel together," from the combination of sentire and com. In modern English, consensual is often used when discussing sex, particularly whether the act was consensual, agreed on by both people, or non-consensual, if one person was unwilling or too young to legally give consent.