The Boston Matrix helps organisations decide which brands within a portfolio they should invest in.
A big, established brand in a declining market should be left to pasture. The profits it generates should be invested in brands with a brighter future - the rising stars. If a brand is struggling to gain share in a growing market, the priority should be to reinvent the offer to exploit the opportunity. A brand with low share in a declining market should normally be left alone to lie down and have a sleep.
This short video explains the matrix in more detail.