Sometimes a composer needs to add length to the symbol for a note.
Perhaps to make a 2 beat note last for 3 beats.
To do that the composer puts a dot after the note.
This makes the note longer by a half of the note’s length.
More Maths in Music
If it is a 4 beat note, adding a dot will mean add 2 beats, which will make it worth 6 beats.
If the note is a 2 beat note, adding a dot will add 1 beat and make it a 3 beat note.
If the note is a 1 beat note, adding a dot will add a 1/2 beat and make it a note worth 1 and 1/2 beat.
Dots as Pets
Just for fun you could think of a dot after a note like pet following its owner and sharing food. For notes time is food.
The different figure on the bottom of the time signature means these notes have all been given 2 beats of time.
Because they have a dot as well they must be given 3 beats.
2 beats of time for the note and 1 beat of time for the dot.