BuddyPress.org

In the BuddyPress plugin there is a folder bp-themes. In this folder is the default BuddyPress theme. The reason to use a child theme is to not edit these files making upgrades as easy as a few clicks.

A child theme is a WordPress theme that has a parent. The child theme gets all it’s files from the parent. If you want to override a file then you copy the file and folder structure over to the child theme.

Example: say you want to ad something to the header. instead of going into the BP plugin folder and changing something you would copy the header.php over to the child theme and edit it. Since it’s a child theme it will override the same file in the bp-default folder.

Read this on how to create a child theme. https://codex.buddypress.org/theme-development/building-a-buddypress-child-theme/

Another issue is if you are not using a BP enabled theme then you will have to use the BuddyPress template pack to get the corresponding files and functions working on a standard WordPress theme.