You are correct about the diameter and the second number is pitch in inches and in this case 3.8 inchs. What that means is this is the distance the propellor would move forward during one rotation of the prop operating in in a theoretical solid (in otherwords no loss of efficiency due to slippage and other factors).

A lower pitched propellor say a 5P for example is also sometines called a fine pitch prop. A higher pitched prop say a 6P is sometimes called a course prop. Just like threads on a bolt.

John