A child theme is a copy of an original parent theme. You can edit the child theme without altering the parent theme. You can create child themes from default HubSpot themes, Asset Marketplace themes, and custom themes.
So, if you need advanced customizations for your website it’s better to use a Child Theme. In general, Child themes are the recommended way of modifying an existing theme.
Check on this article the official way to create a child theme in the HubSpot Developers helpdesk.