What does person-month mean in development time?

I have a question about communication channels that is talking about person-month but I don't understand what is a person-month. Does it mean this many persons? or this many months?

Here is the full context:

Based on statistical data in a company, communication path between two persons consumes 5% of each person’s time (during a 8-hour work day). For a project that requires 12 persons-months of development time, how many people will be needed to finish the project in four-months if (a) the democratic team structure is used and (b) the chief-programmer team structure is used. If the team consists of four persons, what is the difference in the completion time for a team using the democratic structure and a team using the chief-programmer structure?

Person-month is politically correct synonym for Man-month.

It's mean amount of work performed by the average worker in one month.

So, if:

• project requires 12 persons-months of development time
• all team members do only pure development activity (i.e. they are telepaths and they don't need to spend time for communication with each other). [note: this is not your case. In your case developers spend some time (5%) for communication.]

than:

• 4 developers will spend 3 months for 12 persons-months project.

ProjectScopeInPersonsMonths / NumberOfDevelopers = NumberOfRequiredMonthsForProject

• 4 months required for 3 developers to finish 12 persons-months project.

ProjectScopeInPersonsMonths / NumberOfMonths = NumberOfRequiredDevelopersForProject