Hi @AYankova and welcome.
@PawelP explained the reasoning behind it well. Creating a Child Theme is simple, but to help speed things along for you a little bit, you can download a copy of the Ultimo Child Theme below.
When you install and activate it, you won’t notice a difference between the Parent or Child theme. The Child theme sits on top of the Parent theme. Therefore, you can make changes to your theme by editing/adding the Child Theme files. If you edit the Parent theme directory, then you will lose all of your changes when you update the theme. Wordpress also uses the same concept.