Governments and central banks like there to be "just enough" growth in an economy - not too much that could lead to inflation getting out of control, but not too little that there is stagnation. Their aim is the so-called "Goldilocks economy" - not too hot, but not too cold.
One of the main tools they have to control growth is raising or lowering interest rates. Lower interest rates encourage people or companies to spend money, rather than save.
But when interest rates are almost at zero, central banks need to adopt different tactics - such as pumping money directly into the economy.
This process is known as quantitative easing or QE.
The central bank buys assets, usually government bonds, with money it has "printed" - or created electronically these days.
It then uses this money to buy bonds from investors such as banks or pension funds using this "new" money, which increases the amount of cash in the financial system, encouraging financial institutions to lend more to businesses and individuals. This in turn should allow them to invest and spend more, hopefully increasing growth.