Take a quick, deep breath from your diaphragm every time you see this mark :. Be careful not to lose your timing when you breathe. Counting as you play should help you become more confident with this. Breathing technique is discussed in detail in Lesson 5.
If you look at the following example you will notice a line of letters and symbols above the staff (Cm, G7, Cm, etc). These are chord symbols which indicate the harmony to be played by accompanying instruments such as keyboard or guitar.
The big numbers 1 and 3 tell you to play the note. The small numbers 2 and 4 tell you to sustain it until the next note. Notice that there are four beats in each bar.