4.9  ARMY RESERVE   In addition to the armies on the map each country also has an "army reserve". These are armies which are available to placed during the turn. New armies raised with the ARMY action (see 5.3) are added to your reserve. When armies are dispersed they are placed in your reserve ready to be deployed again. Armies in your reserve can be placed on the map with the RESERVE action (see 5.5).
When you build new armies they're usually placed in your reserve. Once you've put some in your reserve you can't take any out until the following turn. Tthis means you must deploy any reserves before you buy any new ones.

4.10  NAVIES   Naval forces are represented as a number of ships. Ships on the map can be either "in commission" or "laid up". If a ship is laid up then it's just a piece of hardware with no crew, no stores and no ammunition, and these are the things you need to add in order to put it into service.
You can build new ships in a naval base with the BUILD action (see 5.10) but each base can only do this once in each turn. Newly built ships are added to the total of laid up ships.
Once you've built new ships (or recovered some from your reserve) you can't commission any in that naval base any until the following turn. This is the same restriction as for army reserves, except that for ships it applies to individual naval bases, so that what you do in one base doesn't have any effect on what you can do in another.
Ships that are laid up can be put into commission using the FLEET action (see 5.14) or they can be scrapped using the SCRAP action (see 5.15). They can't do anything else. Ships must be in commission in order to move and fight. Ships in naval bases can be taken out of commission using the LAYUP action (see 5.12). 
The reason for the two-stage process is to reflect the planning and investment that's need to build up a navy - without it you'd be able to build a fleet and put it into action quicker than you can with army. For the game to provide a reasonable feel of naval strategy in history we need a system that reflects the ability of powers to react to naval construction by their rivals.

4.11  NAVAL BASES   Naval bases are located in coastal land spaces. New naval bases can be built with the BASE action (see 5.16) and unwanted bases can be closed with the CLOSE action (see 5.17). When you build a new base you can't use it until the start of the following turn.
Ships can enter and leave a naval base only from and to the sea space designated as the entrance for that base.
Building naval bases and a navy is a slow business. If you haven't already got a base where you want one, then you have to build it. Then you have to wait until the next turn before you can build new ships in it, or place ships recovered from your reserve (which must have been built already). After that you have to wait another turn before the ships can be put into commission.
Naval bases can be blockaded by opposing navies, who can sit a bigger fleet outside the entrance and prevent your fleet from getting out (or if it already got out, they can prevent it getting back in). Note that blockades only prevent the ships from moving without fighting. The blockaded fleet can always come and out and fight, and a blockade doesn't prevent ships already at sea from tracing a route from their base or drawing supplies from it (for now, but this might change later).

4.12  NAVAL RESERVE   Your naval reserve works in a similar way to your army reserve, except that ships from your naval reserve can only be moved to the map by being placed in naval bases. Use the NAVY action (see 5.11) to increase your naval reserve. Ships from your reserve can be placed in naval bases with the RECOVER action (see 5.13).

4.13  AIR FORCES   Your air force in World Empires is represented by your Air Strength. You don't move squadrons of aircraft around the map. Instead your air force turns up whenever you're involved in a battle (but the number that can get into action is limited by the forces on the ground). Use the AIR action (see 5.19) to increase the strength of your air force.

4.14  AIR STRENGTHS   The attacking air strength available in a given attack is limited by the number of attacking armies. The defending air strength is limited to the number of defending armies, forts and population (added together).
Air forces are not used in Medieval Empires and European Empires.