Remember, the term Microspeak is not tightly scoped to mean jargon used only at Microsoft. It's jargon used at Microsoft more often than in general usage. Today, it's a term that you really need to master if you want to talk with others about project planning.

To book a feature is to commit to implementing the feature, including assigning resources to get it done. This means finding designers to design the feature, developers to implement it, and testers to test it, as well as (the hardest part) finding time in the schedule to do it. The resource that is in shortest supply is usually time, since there is no way to create more of it.

More generally, a resource is booked when it is committed to doing something. This is a natural extension of the concept of booking a room in a hotel or a seat on a train.

Here are some citations.

We will be using the Widget framework that Bob is booked to finish in July.
There are no resources booked for enhancing the Widget framework this release cycle.
The Widget team knows that we need this feature from them, but they haven't booked it yet, so we need to develop a fallback plan.

The term is in general use, but for some reason, Microspeak uses it almost exclusively to describe the commitment to completing a particular piece of work by a particular date. Instead of saying that the work is committed or scheduled or confirmed, we say that it is booked. (If you use one of the other words, people may ask for clarification. For example, if you say that it is committed, some people might think you mean that the change has already been submitted to the source code repository.)

Curiously the antonym of booked is not unbooked. If a feature has no resources assigned to it, the preferred term is unfunded.