SOP - stands for "Standard Operating Procedure". This refers to a well defined document that is used as a "how to" tool. This does not mean that it is used in the context of a simple tutorial tool, but more as a control directive on how to do things appropriately - whether it is taking inventory, or stepping in and out of a laboratory.
While it helps a lot in teaching and training employees, it also helps the company to standardize practices, so that adherence to quality is achieved easily.
Read the associated page (by clicking on the link) for further information.
If you work in the medical devices/biotech industry expect to be reading, training and sometimes authoring your own SOPs.