What does the number for indexes like the Dow or Nasdaq mean?

Shanghai Joe: What does the number for indexes like the Dow or Nasdaq mean?
I understand that the Dow represents the top 30 companies in the US and so forth but what do the numbers mean and how are they calculated.

For example:

NIKKEI 22513,334.76
DOW JONES INDUS. AVG11,370.69

Thanks for answering. I’m new to learning this

Answers and Views:

Answer by what?
the actual number is fairly arbitrary.

it’s like asking why water freezes at 32 degrees.
it’s because the scale had to start somewhere.

what’s important are the changes, which can be used as a barometer for the health of the market.