Getting geared up
If at all possible wait until you have a lesson and allow your instructor to recommend a rod that is well suited to you. Your hand size, physique and overall muscular skeletal reflex are all relevant. Later as you gain proficiency you can adapt your cast to a new rod regardless of its action, however not so much at this stage of the game.
Nothing worse than learning to cast the fly with something that "just plain" doesn't suit you, or was really a good deal but useless for
the task at hand.
Accelerated from a fixed point (forward) to another fixed or stop point (Rear), there is a rapid transfer of energy which throws the line out away from the rod- the pull of the line flexes the rod and loading is achieved.
Fly-rods are designed to flex (bend) as the combined weight (of the feet/inches/cms) of the Fly-line which extends beyond the tip of the rod and is moving away from the tip either in a forward (forward cast) or rearward (back cast). Much like the building and storing of energy in the limbs of a bow as an archer pulls the strings and flexes the limbs, there is a building and storing of energy within the rod as it flexes in response to the extension of the line.
More on this later.
Rods are described generally with the following terminology:
SLOW - A soft or easily flexed (loaded) rod with a given amount of line in play
MEDIUM - A firmer or stiffer rod than the slow rod, requiring either more energy or more line in play to flex (bend or load) the rod.
FAST - A very stiff rod requiring considerably more energy and or line or both to achieve effective flex (bend or load) of the rod.
There are numerous variations on the basic three rods types described above, do not spend a minute at this stage of the game worrying about those.