It means the message length, padded to 64 bits, with the bytes appearing in order of significance. So if the message length is 37113, that's 90 f9 in hex; two bytes. There are two basic(*) ways to represent this as a 64-bit integer,
00 00 00 00 00 00 90 f9 # big endian
and
f9 90 00 00 00 00 00 00 # little endian
The former convention follows the way numbers are usually written out in decimal: one hundred and two is written 102, with the most significant part (the "big end") being written first, the least significant ("little end") last. The reason that this is specified explicitly is that both conventions are used in practice; internet protocols use big endian, Intel-compatible processors use little endian, so if they were decimal machines, they'd write one hundred and two as 201.
(*) Actually there are 8! = 40320 ways to represent a 64-bit integer if 8-bit bytes are the smallest units to be permuted, but two are in actual use.