What does the term 'glycogen mobilisation' mean?

I think the key to understanding mobilization in this context is in the second sentence:

[Glycogen] can be broken down to yield glucose molecules when energy is needed.

The authors are using mobilize in a figurative sense to suggest that glycogen can be quickly and efficiently metabolized to suit the energy needs of the body, like an army may be mobilized to fight a war. See definition #2 on Wiktionary:

mobilize (verb)
2. (transitive) To assemble troops and their equipment in a coordinated fashion so as to be ready for war.

I think that @acvill more or less gets it, but I wanted to add a direct usage in context that makes clear the meaning.

I found this as the top hit when I google "glycogen mobilization". It has a lot of information and it is not straightforward, but it makes clear that mobilization is the process of breaking down glycogen into glucose. For instance, one slide says:

Glycogen phosphorylase usespyridoxal phosphate (PLP) a derivative of pyridixine (vitamine B6) as a coenzyme. B6 is required for the mobilization of glucose from glycogen.

As far as I can tell, "glycogen mobilization" means the mobilization of glucose from glycogen, therefore it is a special case of glucose mobilization in which the source molecule is glycogen. For example:

Glucose is a negative regulator of liver phosphorylase. – Glucose is not mobilized when glucose is abundant.

Additionally, wikipedia says:

...debranching enzymes mobilize glucose reserves from glycogen deposits in the liver.

and

glycogen debranching enzymes function in glycogen breakdown and glucose mobilization. When phosphorylase has digested a glycogen branch down to four glucose residues, it will not remove further residues. Glycogen debranching enzymes assist phosphorylase, the primary enzyme involved in glycogen breakdown, in the mobilization of glycogen stores.

So glycogen mobilization == glucose mobilization, in the context of human biology. That is admittedly kind of confusing.