Microbiology is the branch of life science that concentrates on very small life forms. In Microbiology you would study bacteria, viruses, and microscopic invertebrates. Many microbiology courses are framed around studying the organisms that cause human diseases.
In a microbiology class you would refine your microscope technique, and you would learn some other standard laboratory techniques, including sterile procedure for specimen collection and handling, how to set up and analyse cultures, and how to identify organisms that are too small to identify visually.
Microbiology is an interesting class; if you enjoyed biology, you will probably like it. As #2 points out, there are lots of well-paid jobs that involved microbiology skills.