Understanding the length b used in MD5
what does 64-bit representation stand for?


It's the length (in bits) of the hashed message, expressed as a 64 bit binary value (in little endian order).  If you hash a 1 character message, $b=8$ (as 1 byte == 8 bits); this would be represented as the 8 bytes 08 00 00 00 00 00 00 00 (the 08 is first, because in little-endian order, you place the least-significant part first)


  what does lower-order bits mean?


The length of the the $b$ field within the padding is 8 bytes long; if you were to hash an extremely long message (one $2^{61}$ bytes or longer), it might be the case that $b$ wouldn't fit in an 8 byte field.  In that case, what you place into the field is the lowest 8 bytes into the field; another way to express this is that you place the value $b \bmod 2^{64}$ into the padding field.

On the other hand, $2^{61}$ bytes is an enormous amount of data; it is approximately the amount of data that flows over the internet globally in a day.  It is quite unlikely that you (or anyone else) will ever compute an MD5 hash on that much data; you can in practice ignore this sentence.