This was a fun and rousing song that was used to urge people to buy as many war bonds as possible to support the war effort. In fact, the first time it was used was in a Bugs Bunny cartoon but it became so popular that real singers started using it during bond drives.
The tall man with the high hat and the whiskers on his chin
This means "Uncle Sam" which is a cartoon representing the country of the USA to its citizens. In effect your country is asking you to buy stamps and bonds to support the war effort.
If you couldn't buy war bonds straight out you could buy various denominations of war "stamps" which you placed in a booklet. When you have enough stamps you could trade them in to buy the war bonds.
