A dot after a note takes the original time-value of the note and lengthens it by half:
Figure 1: A dotted note.
A rest may also be dotted:
Figure 2: A dotted rest.
Double Dotted Note
A double dotted note lengthens the original time-value by three-quarters:
Figure 3: A double dotted note.
Triple Dotted Note
A triple dotted note lengthens the original time-value by seven-eighths:
Figure 3: A triple dotted note.
Also see Note & Rest Values | Tie.