What is security guard mean?

It is important to understand both terms ""Security" and "Guard"

1. Security means "Security is the degree of resistance to, or protection from, harm. It applies to any vulnerable and valuable asset, such as a person, dwelling, community, item, nation, or organization." or "a form of protection where a separation is created between the assets and the threat."
2. Guard means "a person or group of persons that guards, protects, or keeps a protective or restraining watch." or "something intended or serving to guard or protect; safeguard: insurance as a guard against disasters."

Now if you combine these two, security guard is a person who "a person or group of persons that guards, protects, or keeps a protective or restraining watch with intention of providing degree of resistance to, or protection from, harm. It applies to any vulnerable and valuable asset, such as a person, dwelling, community, item, nation, or organization"

I hope this answers your query : )

Those are two questions in one and one is what is security while the other is what is a guard or probably you intended to ask what is the role of a security guard?

security is a very wide topic and involves lectures and lectures on types of security, means, modes, how is it performed, what it involves , and the list is endless. The simple definition could be how can I safeguard my property, myself from being vulnerable to outside interference and or vandalism/theft/calamities induced by persons or by nature.

Guard could  mean to be able to oversee the security of a certain building, place or safeguard against loss/disappearance of items or stop an intruder from gaining access or even to guard your body against harmful diseases etc

It is simply , he is the one who is securing and protecting the origination's assets (people, properties, and information )