A normal menstrual cycle lasts around 28 days, an irregular period can mean that it is coming too early or too late.
There are a number of different reasons that might be causing your period to be irregular. It might be that they are normal to you. If you are concerned about your period then you can go and see your GP. Some of the most common causes include:
Puberty - your first few years of periods may be irregular as your body gets used to the change
Menopause - this will usually start after the age of 45
Extreme lifestyle changes - extreme weigh loss or gain, over exercise and/or stress
Pregnancy - if you are worried about this then it is a good idea to have a pregnancy test
Medical condition - conditions such as polycystic ovary syndrome can lead to irregular periods