|A "stack" is the set of programs and/or system services needed to supply a given end-user (in this case) service.
As I understand it, for bluez there's
User programs : e.g. hcitool
System software : e.g. whatever system processes are needed to monitor/provide bluetooth services
System libraries : e.g. libbluetooth this provides interfaces for both system and user programs
Kernel module : e.g. bluetooth module, which provides the kernel side implementation details
As you can see, this forms a "stack", with each layer needing services from the layer below it to provide the services it needs to the layer above.
Keep Calm and Carry On