This is the 11911th most frequent Polish word.
ktorzy
"Którzy" means "who" or "which" (used for plural masculine or mixed gender subjects).
In this sentence, 'którzy' introduces a relative clause specifying the subset of 'Ludzie' (People) described by loving to help others.
Ludzie, którzy kochają pomagać innym, są zawsze mile widziani.
People who love helping others are always welcome.
In this instance, 'który' creates a specificity about 'Niektórzy' (Some people) who were on vacation, forming a detailed narrative.
Niektórzy, którzy byli na wakacjach, opowiadali ciekawe historie.
Some people who were on vacation shared interesting stories.
Here 'którzy' connects 'Nauczyciele' (Teachers) to the characteristic of applying new teaching methods by employing a relative clause.