What is the physical significance of 'a' in the Kepler orbit equation?

[Jerbearrrrrr]

Sorry if this is in the wrong forum.

I have the equation $$r = \frac{a(1-e^2)}{1+e \cos \theta}$$,
and I'm wondering what the physical significance of the numerator is.
More specifically, what is 'a' (since e is what it usually is)?

I've seen various other representations with terms like angular momentum on the top (or rather h^2/GM).

In the context of what I'm doing, it's written this way to (presumably) uncover any implicity 'e'-dependence in the orbit.

 

[Janus]

'a' is the semi-major axis of the orbit, or the average value of 'r' for the orbit.