When a dot is placed immediately to the right of a note, it tells us to increase the length of that note, by Half of it's Original Value.
In other words, a Quarter note lasts for one count, but when it has a dot after it, we must hold it down for one and a half counts.
If you remember the dotted half note, from earlier, you will know that it is held for three counts.
The half note is normally two counts, but the dot tells us to add half of its value, (one count) so making it three counts long.
The same applies to a whole note ( four counts) The dot adds half of four counts ( two counts) making a dotted whole note, six counts long.
Dotted Quarter notes are very often followed by an eighth note.
This all may sound very confusing, but the following audio and video examples, should make everything more clear, and will show how these notes are used when playing music.