A "bootable clone" is a complete copy of your startup volume. You can boot up your Mac from it and run normally (if a bit slower), if your internal HD fails (unlike Time Machine, which you must
restore onto a drive or partition).
In most cases, however, it does not contain copies of previous versions of things that have been changed or deleted from your system, so you can't recover earlier versions of lost, changed, or corrupted files. And since they take so much more CPU than Time Machine's quick hourly backups, it's usually not practical to run them more than once a day.
Many of us keep
both a "clone"
and full Time Machine backups, to get the advantages of both, and "just in case" something goes wrong with one of them. That's why it's best to put them on two separate disks (but you can use two partitions on the same disk).
The most common apps for this are
CarbonCopyCloner (which I use) and the similar
SuperDuper! CCC is donationware, so you can use it for a while, then send them some $$ so they can keep it up to date. SD has a free version you can try, but to do an update (vs. a full replacement), or automatic scheduling, you need the paid version (about $30, I think.)
