Biotechnology is used to describe quite a big area of science, but generally, the idea is to use biological systems to develop useful products. This can be trying to develop medical tests, trying to isolate useful products from plants and animals or doing genetic science.
When I was working in the biotechnology lab, I was developing methods to test for dangerous chemicals in waterways. We were using antibodies, which are part of your immune system which are designed to recognise particular molecules. The chemicals we were testing for are thought to cause some kinds of cancer.