The book I'm going through doesn't seem to explain a lot of details. I'm looking at the next example of sheet music but the author doesn't mention or explain a note with a dot next to it.
A dot following a note, as in your diagram, increases the length of the note by one half or 50%. So without the dot it would be a quarter note (duration 2 eighth notes) and with the dot it is a quarter note plus an eighth note (duration 3 eighth notes).
As @arminb says you can google this and get lots of answers.