Could you pick up a boulder with a pair of tweezers? Could you pick up a piece of hair with a bulldozer? Having the right size tools for the job can be pretty important. When scientists study the nanoscale, they need to use tools that are the right size. To study things that are very, very small, they need very, very small tools.
Scanning probe microscopes (SPM) are a set of tools with very small parts that help scientists image the nanoscale. Each type of scanning probe microscope involves a very fine probe tip that scans back and forth over a surface. The first SPM was the scanning tunneling microscope (STM). STM is limited to surfaces that are electrically conductive. Later, Atomic Force Microscopy (AFM) was invented, which could scan any surface, conductive or not.