It represents an overall picture of financial flows between the UK and the rest of the world. This parameter consists of a large number of components that take into account all types of cash flows to and from the country. In fact, it is the difference between all cash that entered and left it.
Growth of the balance of payments deficits affects the rate of the national currency, because it means the outflow of funds, i.e. reduce of foreign investment, falling confidence in the country, etc.