Erase Filesystem block error

I was downgrading the firmware from 1.6b4 because the RTC not working to 1.5b6 and when I try erase filesystem I get the message:

*Block 147 is bad* 
*Block 1703 is bad*

What exactly it is mean?

Dear @KriSS

At a first glance, this error message might look critical, but actually it is nothing to worry about.

As you probably know each NAND flash device can have a certain percentage of blocks which cannot store data reliably, they are called bad blocks.

If this message appears while erasing the flash file system, it only means that the system detected two such bad blocks in the flash area which is reserved for the file system.
The blocks are automatically replaced by good blocks, therefore no action is required.

Regards, Andy