Essentially a child theme is a theme which can be used along side your main theme allowing you to separate custom design changes. This is important because it allows you to update your main theme whilst maintaining your design changes.
Do I need a child theme?
For 90% of users the answer is - no. A child theme is really only needed if your making substantial changes to the core theme code. If your making design changes using Elementor or via the admin area, you do not need to create a child theme.
Where can I learn about child themes?
Here is the official link to the WordPress documentation on child themes;