What does the values or details mean?
Most electronic chargers are constant-voltage sources. A typical phone charger has a voltage that it outputs (5V in this case, which is what USB uses). Additionally, there is also a maximum current that the charger can supply (and still maintain voltage regulation). Basically, the two numbers together give you a power rating for the charger. If you attempt to short-circuit the output, or drive it through a load that has less than about 16 ohms (for the 300mA figure), the supply voltage will droop below 5V.

In summary, for a charger with 5V/300mA spec, you will get 5V out of it constantly and up to 300mA.