Mental capacity refers to a person's ability to make decisions and understand the consequences of those decisions. It involves cognitive functions like reasoning, memory, and comprehension. When someone has full mental capacity, they can weigh options, understand information, and communicate their choices effectively. However, mental capacity can vary from person to person and can change over time due to factors like age, illness, or injury. For instance, individuals with conditions such as dementia may experience a decline in their mental capacity, affecting their ability to make informed choices about their lives and health.