If you are using a stable release of the Debian distribution, you know that it contains only packages of a certain version. For example, php 5.6. And, until the distribution kit in this thread is updated, you will not be able to install a more recent version of the software ...
... Until you connect an additional, but official repository of backports packages. It provides newer versions of certain packages. For example, if nginx version 1.6.2 is located in the standard repository, then from backports you can install version 1.9.10, which includes many necessary improvements.