I study psychology at Uni but didn't do the A level so I'm not entirely sure.. the question is quite vague.
But the biological approach means how genetics (or your genes) influence behaviour, we know that behaviour is a combination of both genes and environment which gives rise to individual differences in behaviour. e.g. some genes make you more likely to get depression or mental illnesses and some genes make you more impulsive or rash so your more likely to partake in risky behaviour or get addicted to drugs.
Another aspect is that innate biology drives behaviour, it isn't just a product of the external environment but behaviour could be the product of the internal environment i.e. hormones, hormonal imbalances such as testosterone and stress can drive behaviour by influencing the brain's reward pathways to take drugs. i.e. food behaviours like anorexia could be driven from within, a lack of hormones as well as social factors. Stress can even cause depression, not all stress is caused by the environment it could be a biological mechanism that has gone wrong. therefore, depression could be caused by nothing but the body itself.
It is clear that behaviour is the culmination of environment and genes and biology takes the genetics bit and hormones into account. sometimes brain structures, such as the size of certain brain areas also governs behaviour such as the amygdala etc. Biology encompasses all this.. think of it this way some behaviours are passed down by your parents, the way they do things you do things and biology kind of helps to show this by twin studies to explain which bits of behaviour are caused by the passing of genes and which are caused just by the environment.