WP_DEBUG_LOG

WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).

Note that this allows you to write to /wp-content/debug.log using PHP's built in error_log() function, which can be useful for instance when debugging AJAX events.

define( 'WP_DEBUG_LOG', true );

Note: for WP_DEBUG_LOG to do anything, WP_DEBUG must be enabled (true). Remember you can turn off WP_DEBUG_DISPLAY independently.

WP_DEBUG_DISPLAY

WP_DEBUG_DISPLAY is another companion to WP_DEBUG that controls whether debug messages are shown inside the HTML of pages or not. The default is 'true' which shows errors and warnings as they are generated. Setting this to false will hide all errors. This should be used in conjunction with WP_DEBUG_LOG so that errors can be reviewed later.

define( 'WP_DEBUG_DISPLAY', false );

Note: for WP_DEBUG_DISPLAY to do anything, WP_DEBUG must be enabled (true). Remember you can control WP_DEBUG_LOG independently.

Please find my reply below:

When I activate WPFORO plugin, the caching seems to stop.

Why you think it stops? wpForo only work on forum page, it doesn't affect other pages and website parts. wpForo doesn't stop any cache, please disable all your cache plugins for wpForo page, it has own cache solutions. Exclude the /community/* pages from caching.

Here is the official varnish maintainers plugin:
https://wordpress.org/plugins/varnish-http-purge/

Here is my official thread on support:

https://wordpress.org/support/topic/problems-with-varnish-caching-on-dreampress/