Explanation /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
/bin/bash executes a new instance of bash
-c passes one or several commands to be executed
$(...) runs the command within () and returns its result (Command Substitution)
curl ... gets the file specified in the URL

So basically bash gets called to execute the content of the file fetched by curl.
Some things you may try to understand this in more detail

run curl ... to see what curl returns, or curl ... | less to examine it in more detail
run /bin/bash -c 'echo &quot;Welcome to bash&quot;'
read man bash and man curl