I think it's quite common to do this, and highly advise it. (I wasn't allowed to).
A unit is a semester of that subject. So originally, VCAA intended units 1 and 2 to be year 11, and units 3 and 4 to be year 12. Only scores from Units 3/4 count to your final score for the subject (in your ATAR); in other words, the work you do in year 10 will not actually count to your final score, but the unit 3/4 subject you do in year 11 will.
See Year 12 subjects in Year 11: A guide to starting VCE early.
Of those subjects, I've done Health and Human Development, which I really enjoyed. In my opinion, it would be FAR easier than history, somewhat easier than psychology, and I've no idea compared to ICT; I personally think it's quite an 'easy' subject well suited to year 10/11.
click here for a HHD course overview - unit 3/4 courtesy user Joseph41
Units 1 and 2
I don't remember very well, but from memory you learn about: physical, social, emotional and intellectual development; health; factors that increase or decrease risk of diseases; healthy eating; and how we develop over the lifespan (prenatal, childhood, youth, adulthood); nutrition.
Note: AOS means 'area of study', which just means a particular chunk or section of the year's work.
Unit 3: Australia’s health
AOS 1: Understanding Australia’s health
AOS 1 includes health, and why it is important; the health of Australians compared to other nations; variations in health within Australia, and reasons for those variations; the National Health Priority Areas; and nutrition as a factor that influences health.
AOS 2: Promoting health in Australia
AOS 2 includes different models of health, including models that the Australian Government has used (and uses) to guide its health policies; the Ottawa Charter for Health Promotion; the role of VicHealth; Australia’s health system, including Medicare; the role of the Australian Government in promoting good health; and the role of non-government agencies in doing likewise.
Unit 4: Global health and human development
AOS 1: Introducing global health and human development
AOS 1 includes the distinction between developed and developing countries; sustainability and human development, and why they are important; factors that influence the discrepancy in health between Australia and other nations; the United Nations’ Millennium Development Goals.
AOS 2: Promoting global health and human development
AOS 2 includes the interrelationship between health, human development and sustainability; types of aid, and how and when they are used; the United Nations and its role in global health; the World Health Organisation; priorities of the Australian Government; programs focusing on literacy, food security, HIV/AIDS and malaria, immunisation, safe water and sanitation.
Feel free to ask any specific questions about how the ATAR works or whatever!