A voltmeter showes the voltage in the system, if it's below 12volts then the battery won't be charging and probably going flat, if it's above 12 volts then it should be charging.
If it's an ameter then it will show current flow in either direction, negative means charge flowing out of the battry, positive means it's flowing in.
Genrally speaking, right of centre good, left of centre bad :(
Sounds like an alternator problem to me, charge the battery up now or that too will become damaged.