What is C34387295A on 1000000 dollar us bill?

It's supposed to be a serial number like those found on genuine bills. However your "one million dollar bill" is actually a novelty item that sells for a few bucks in gift shops and online.

The US never printed a genuine million-dollar bill. The largest circulating bill was $10,000. Only a small number were printed and they rarely saw circulation. The largest US bill ever made was the Series 1934 $100,000 gold certificate. A few dozen were printed and were used only for transferring funds between various government departments.

Distribution of all bills higher than $100 was suspended in 1969 in an effort to limit their use by criminals and terrorists.