That means each record is 50 bytes long. If 50 of these records were put together in a block you would have a block size of 2,500. Say you had a database with a logical record length of 125 and a block size of 1,250. Four bits make a nibble. Two nibbles make a byte.
Bytes make fields.
Fields make records. And records make blocks. To save space. Coding Spanish, French, and Italian all use the same alphabet. In a similar fashion, there are different kinds of binary code based on a computer's 1s and Os.