The corporation has a fixed valuation. This cannot be changed at will (other than by doing very stupid things to reduce the value, or doing very intelligent things to increase the value).
A "share" is an ownership fraction. If you issue 10 shares of stock total, then each share represents a 10% ownership stake. If you issue 100 shares, each share represents 1% and so on.
You can have shares less than 1%, and in fact this is very common. Also, the shares do not have to indicate any whole amount in dollars or cents. So you can create 1,000,000 shares of stock for a company that has a total net worth of $100 and issue them appropriately.
Now, if you originally issued 100 shares, and allocated 20 of them, and then you create another 100 shares of stock, you now have a total stock pool of 200 shares. But, the company valuation has not changed at all, so each share is now worth .5%, and the relative value of those 20 shares issued to people will have dropped by 1/2 (they now have an ownership stake of exactly 1/2 of what they originally had in a corporation whose value has remained constant).
Sometimes when issuing new shares an additional allocation is made to previous stock holders so that the dollar value of their stock remains constant.
That's a brief explanation, I hope it helps.