This is a recurring question on the forum and I have written an extensive answer in simple words here:
To sum it up: An older PHP version (5.6) has an bug which causes PHP to do unexpected things when always_populate_raw_post_data is enabled. Even when you disable it (set it to 0) php still shows (wrong) warnings which may break piwik.
To avoid this you need to set always_populate_raw_post_data=-1 in the php.ini.
Another (and maybe even better solution) would be upgrading to php 7 or later as this also avoids the issue.