This is the number the game uses to identify your airline. You need this number when, for example, ordering planes on the forum so other airlines can send the plane to the private used market. Also, when posting bug reports, you need to write down your airline ID. You can find your ID on the Admin Panel.
DOC (Daily Operation Contribution) is, quite simply, daily profit. It is the sum of all of your route revenue and miscellaneous revenue (e.g., food and drink sales) minus your miscellaneous expenses, staff expenses and fuel expenses. Given that one actual hour is equal to one game day, you will have 24 of such events every actual day.
At midnight of the first day of the game month, you will be required to pay all monthly fees. These fees include maintenance fees, gate rental fees, and lease payments. If your DOC is higher than all monthly fees, then you will realize a profit at the beginning of the game month (albeit most likely smaller compared to other days).
If your DOC is lower than all monthly fees, then you will realize a loss at the beginning of the game month. This may put you temporarily in debt. As long as your DOC can recover from such debt--that is, get you out of debt--before the next month, your airline can operate normally.
The number of return flights per day. To select a frequency when creating a route you need slots which is automatically bought when you rent a gate. An 0.5 frequency means the aircraft flies out one day, back the next.
A gate at an airport allows an airline to start flights to the airport. Each gate gives 10 slots. At the end of each month the price of each gate is taken out of the airline's account as rent. See Rent Gates
A lease basically means renting an aircraft. The leasing cost is paid at the end of each month.
A loadfactor (LF) is a percentage showing how full your aircraft is.
OCP (One Click Purchase) allows you to purchase a gate without a confirm or quantity select page. The OCP button is located to the left of the regular Rent Gates button.
Return On Investment (ROI). The amount of time it takes for you to earn back the money you paid in the first place. An aircraft bought for €1,000,000 making €100,000 DOP will have a ROI of 10 days.
Each gate at an airport has ten slots, each slot gives the ability for an aircraft to arrive and depart from that gate. That means each gate can have ten daily flights. These can have different frequencies as long as the total number of departures is less than ten (e.g. seven routes 1x daily and one route 3x daily use all ten slots). Please note that any flight that has half a frequency uses one slot: 0.5 uses one, 1.5 uses two, 2.5 uses three and so on.
Tokens are used to join and create worlds and purchase premium features. You get 40 tokens on registration and get one token every 12 hours. Entering a world costs 30 tokens (however, if you are not in any worlds, and you have less than 30 tokens, you can join a public world for however many tokens you have).