This is all part of a feature provided by Intel (and more recently AMD) on their x86 CPUs which allows the firmware (or OS) to control power consumption limits for the processor, thus allowing better control of system power consumption.
The 'long duration' power limit places an upper limit on the power consumption of the CPU over extended periods of time. Your understanding is correct that the 28 second time window is how long the CPU can run over that limit. Most CPUs can in fact have this safely set above their TDP, but the cooling system needs to be able to handle a TDP equal to this limit to avoid causing damage to the CPU.
The 'short duration' power limit puts an upper limit on how much power the CPU can use when the long duration limit is not being enforced. I'm not entirely sure myself what 'unlimited' means here. It could be a marker that this is applied at all times (IOW, you can't go over this limit), or that this limit is just unused (not likely).
In both cases, 'unlocked' means that the values can still be changed by the system (or firmware). Most CPU configuration items like this can be 'locked' once set by writing a 1 to a specific bit in another register to prevent those values from being changed until the CPU is reset. This functionality is used a lot on embedded systems with very strict power or thermal envelopes to ensure that user software cannot cause problems for the user.
The third listed limit is, I believe, upper bounds for what you can set for the time windows and power limits. The fourth should be the physical upper limit on power consumption actually enforced by the CPU.