What does it mean if a country has a declining population pyramid?

Vital data of any country is generally collected every 10 years through census surveys . The data collected from such surveys are often of immense value for country wide planning or developing policies etc. Internationally data compiled are also used to study the country's advancement, achievements or economic position globally. 

Demographic data are often represented using graphs. are some of these data are more meaningfully displayed in certain types of graphs. Population Pyramids are one such graphical presentation.