This constant is used to add (since SPIP version 3.0.9) a log file (tmp/log/profiler.log) tracing the names of loops (and calling templates) whose calculation exceeds a threshold defined in milliseconds.
To trace the too long loops, it is necessary to define the constant code>_BOUCLE_PROFILER in your file config/mes_options.php (see the article dedicated to it)
example :
// trace loops that take more than 5 seconds to be calculated
define('_BOUCLE_PROFILER', 5000);