They usually contain the remains of a dead soldier who is unidentified, and is thought to be impossible to ever indentify. Much work goes into trying to find a certain soldier, and to verify that it is indeed one of you own soldiers.
The most famous, and earliest, is that in France under the Arc de Triomphe that was istalled after the First World War.
These tombs are also used to represent the unidentified fallen of later wars.
Examples include:
wikipedia.org dumped 2003-03-17 with terodump