1. What does exactly an event (or observer ?) do ?
- RE : In a simple term i would say , when you would like to do your custom actions on before or after the magento's native functionality even you can use event observer , let's say you would like to show a popup when customer is successfully logged in into the system then you can create observer on the event customer_login and then write the code for your popup.
Refer this link for more details - https://devdocs.magento.com/guides/v2.2/extension-dev-guide/events-and-observers.html
2. How to use it in order to alter the result of the product collection in the front store ?
- here i would like to give working example link , refer this - https://magento.stackexchange.com/questions/114207/unable-to-update-product-collection-via-observer-...
Hope it helps !
if issue solved,Click Kudos & Accept as Solution