Cannot understand Block Cipher example
Why encoding 8 characters using 8-bit ASCII results in 800-bit message?  

That is not correct, 8 ASCII characters is 8-bytes times 8-bits per byte which is 64-bits.

The question seems to be: Encoding 100 characters using 8-bit ASCII results in an 800-bit message.

That is 100 ASCII characters is 100-bytes times 8-bits per byte is 800-bits.
What padding means?
Since block based encryption with a block mode such as ECB or CBC means that the input must a  multiple of the block size because the data is processed one block at a time and partial blocks can not be processed.
What last sentence means? (difficult to understand)
Basically answered above.
See Padding (cryptography)
and especially: PKCS#5.


Note: ASCII is one 8-bit byte per character—only.