A skills mapping is an overview of the knowledge and skills in a department or the whole company. It is used to find gaps, plan development, and make sure critical skills do not live with only one person.
The mapping can be done with self-assessment, conversations with the manager, or tests. A mix is often best, because people both over- and underestimate themselves.
The result is often collected in a skills matrix. It makes it easy to see where to focus and which courses or programs to assign to whom.