WordPress parent theme are the main file which contains all data and coding.
If you directly edit everything in the parent theme such as CSS, functions and any other pages then it might be possible to lose many of previously made changes when you will update to new version of that theme. So, it is a good practice to use child theme to be safe from any update.
The child theme does not get updated when you update Parent Theme.
In your case, as you mentioned organicfood.zip is the parent theme which also contains a child theme zip file. You are lucky that already you have child theme. So, you can just start editing in your child theme.
Many WordPress theme does not provide child theme zip file. In that case, you have to create child theme and edit file according to your requirement. So that, next time when you will update your parent theme then you will not loose your design and other changes which you have made in your child theme.