It means metals that are highly reactive. It's one of the most self-evident pieces of terminology in all of chemistry.
Perhaps you were trying to ask which metals are considered "highly reactive", which is a separate question, and one that to some extent depends on your definitions, but would certainly at minimum include all the alkali metals, maybe the alkaline earth metals, and possibly a few selected others (aluminium, for example... we don't usually think of aluminium as "highly reactive" because it immediately forms a thin transparent protective layer of aluminium oxide when exposed to air, but without that layer, it is indeed pretty reactive).
Iron is reactive but not highly reactive, while oxygen is highly reactive. When iron reacts with oxygen, it forms rust. Oxygen is highly reactive because it readily forms bonds with other elements, making it important for various chemical reactions.
Potassium is highly reactive, even explosive in some circumstances.
Gold is highly malleable and almost non-reactive.
Plutonium is not highly reactive under normal conditions. It is a relatively stable element that does not react readily with air or water. However, it can be reactive in certain chemical reactions or when exposed to high-energy particles.
Highly reactive elements easily react with other substances in the environment, forming compounds rather than existing in their pure form. This is why it is rare to find pure samples of highly reactive elements in the ground.
Cerium is reactive but not highly reactive.
the highly reactive metal is potassium
Sodium chloride is not highly reactive.
Tin is highly reactive.
Iron is reactive but not highly reactive, while oxygen is highly reactive. When iron reacts with oxygen, it forms rust. Oxygen is highly reactive because it readily forms bonds with other elements, making it important for various chemical reactions.
No, argon is not a highly reactive gas. It is stable.
Potassium is highly reactive, even explosive in some circumstances.
highly
No
No.
Gold is highly malleable and almost non-reactive.
phosphorous is a highly reactive nonmetal but fluorine is more reactive