Jisho

者 means person and a qualifier means that you have to attach nouns or adjectives before it to make it work. In that sense, you should think of 者 as a suffix instead.

Here's some examples:
医 い means medical, when I attach 者 'person' to the end, it becomes 医者 いしゃ meaning doctor
患 かん means suffer, when I attach 者 'person' to the end, it becomes 患者 かんじゃ meaning patient

You can do this for all sorts of preceding nouns and adjectives. That makes 者 a really important word, so make sure you put this in your memory.

You may ask well what's the difference between 人 which also means person? There's no real difference in meaning, they're just used under different circumstances, like 者 needing a qualifier for example.

Another pronunciation for it is mono. Like in 若い者 【わかいもの】which means young people.