This procedure refers to a removal of the pressure or compression on the nerves (hence, decompression), and the “micro” refers to use of the microscope. This procedure also is usually done in a minimally invasive fashion, through the tubular retractors.
After decompression, there is more room for the spinal nerves.
In this MRI cross section, the narrowing (stenosis) of the place for spinal nerves can be seen. The area indicated by the tip of the arrowhead should be significantly bigger.
After a decompression, you can plainly see that there is more room for the nerves.