By adding a dot beside a given note value, you "1.5x" its time value. It's just a musical piece of grammar that's useful to know.
Example 1: There is a whole note worth 4 beats. If you "dot" that whole note, 4 multiplied by 1.5 equals 6, so the dotted whole note is instead valued at 6 beats rather than 4.
Example 2: A half note is normally worth 2 beats, so when you dot a half note, it's worth 3 beats instead.
Example 3: A quarter note is worth 1 beat, so when you dot a quarter note, it's worth 1.5 beats instead.
Another way of modifying the value of notes is using "ties".
If notes are "tied", you actually play the notes as a single note, and combine their time values.
Ties are especially handy, because you can connect notes across bar lines.
Example 1: You can see two 1/4 notes being tied. You'll simply add their values together (1 beat + 1 beat = 2 beats) and play them as a single held note.
Example 2: Here's a situation where multiple notes are tied, and across a bar line too! Since it's 4/4 time, half notes are worth 2 beats. Therefore, the value of the tied notes is 4.5, played as a single held note.