What are unstable selectors or how to find elements competently (Selenium WD)?

Question

css

Hey! Recently I met with such a concept as "Unstable selector". It is implied that it is bad practice to use such selectors.
For example, this one is unstable (located on yandex.ru):

"button.mini-suggest__button.button_theme_websearch.button_size_ws-head.i-bem.button_js_inited"

Question: what are "stable" selectors, how can I understand that the selector I have chosen is "stable"? And are there any guidelines for finding an element by other ids (like class name, id, etc.)
Thanks!