Is a "start up disk" the same thing as your hard drive? (iMac)

My poor old iMac says its “start up disk” is full and it cannot read or write anything nor do much else.

I am going to delete a bunch of stuff off the main hard drive.

My question is if there is a particular area on the drive called a start up disk area or if it is just the hard drive itself.

Thanks for any responses.