A more thorough explanation:

A reasonable man is a hypothetical person used as a legal standard to determine whether someone acted with negligence. This person exercises the degree of attention, knowledge, intelligence, and judgment that society requires of its members for the protection of their own and of others' interests. The reasonable person acts sensibly, does things without serious delay, and takes proper but not excessive precautions.

For example, if a driver is texting while driving and causes an accident, a reasonable person would not have engaged in such behavior while operating a vehicle. The driver's actions would be considered negligent because they did not meet the standard of a reasonable person.

It's important to note that a reasonable person is not necessarily the same as the average person. The reasonable person's notions and standards of behavior and responsibility correspond with those generally obtained among ordinary people in our society at the present time, who seldom allow their emotions to overbear their reason and whose habits are moderate and whose disposition is equable.

In criminal law, the actus reus of murder (and therefore of any criminal homicide) was declared to be unlawfully killing a reasonable person who is in being and under the King's peace, the death following within a year and a day. In this sentence, the word ‘reasonable’ does not mean ‘sane’, but ‘human’. In criminal law, a lunatic is a persona for all purposes of protection, even when not so treated for the assessment of liability.