Basically, when a MacBook is booted it allows random code to be executed from an attached Thunderbolt device (in a form of a legacy mechanism of Option ROM). The Option ROM is loaded unconditionally, including the case when the host system is rebooted to update its firmware. During such upgrade its primary on-board ROM is writable, so the exploit can write itself in it, replace RSA key used to verify firmware upgrades and thus prevent re-flashing the host with any official updates. Additionally, all this is possible because ROM includes only rudimentary self-integrity checks (in a form of CRC32) and proper crypto-signature checks are only applied during the update and not on every boot.
This particular exploit is pluggable by making firmware not use Option ROM during upgrades, which is a fix being deployed by Apple. Meanwhile you may want to superglue your TB port.