A locked boot loader is one that will only start an operating system and a recovery that "approves." Vendors digitally sign the recovery, kernel and kernel modules, and unsigned binaries are refused to flash and boot. This is the same as Safe start provided by UEFI.
On the PC, one can simply disable the safe boot option if you want to start unsigned binaries. So, why have providers made it so complicated to disable it on Android?
Each provider has its own extended process to unlock the boot loader, such as providing an unlocking tool, an unlock code and the device registration, which ultimately give the same result. The vendors are making it even more difficult, as HMD initially refuses to unlock Nokia's boot chargers and Huawei will stop providing the boot loader unlocking for all new devices.