What is the difference between 'local value' and 'master value'?

master is either the value compiled into PHP, or set via a main php.ini directive. I.e., the value that's in effect when PHP fires up, before it executes any of your code.

local is the value that's currently in effect at the moment you call phpinfo(). This local value is the end result of any overrides that have taken place via ini_set() calls, php_value directives in httpd.conf/.htaccess, etc.

For example,

php.ini:     foo=bar
httpd.conf:  php_value foo baz
.htaccess:   php_value foo qux
ini_set:     ini_set('foo', 'kittens');
.user.ini    foo=bar   # this file works conditionally see https://stackoverflow.com/a/32193087/1818723

Given that, the master value is qux, and the local value is kittens.

"Master Value" (from php.ini) could be overridden with "Local Value" in httpd.conf, .htaccess or other Apache configuration with the php_value directive.

The first is the local value, and the second is the global value. The local value overrides the global value and is set within PHP, HTACCESS, etc., whereas the global value is set within php.ini. To answer your question, the first value is used.