The N+1 problem occurs when a first query loads N rows, then an additional query is run for each one to read a relationship: N+1 queries in total. It heavily slows pages with a lot of data; you fix it by loading relationships in advance with a join or eager loading.
Example
// 1 query + 1 per article
foreach (Article::all() as $a) { echo $a->auteur->nom; }