Monocytes are a type of white blood cell are produced in the bone marrow. They make up about 3-8% of the total white blood cell volume. Monocytes can leave the blood stream and enter other tissues and organs in the body, where they have the ability to turn into different types of immune cells called macrophages and dendritic cells.
Once monocytes have turned into macrophages, they are part of the innate immune system, which means that they "non-specifically" destroy any invaders they encounter in the body, such as bacteria and parasites. Non-specifically means that monocytes, after evolving into macrophages, do not have to recognize the invader specifically, but instead simply recognize the invader as something that should not be present and should be destroyed.
Once monocytes have evolved into dendritic cells, they assist a part of the immune system known as the adaptive immune system. This is also referred to as "acquired immunity." Dendritic cells don't destroy invading bacteria and other microbes directly. Instead, they identify something as foreign and share this information with other immune cells called B cells and T cells. This helps the T and B cells learn to recognize the foreign invader and respond more strongly to destroy it the next time it is encountered.