::1 is for loopback address. An IPv6 address contains 8 16-bit pieces.
What do this representation
1001::1 64 mean (what is the full address with all 8 16-bit pieces)?
This is explained in the IPv6 Addressing Architecture RFC (RFC 2373) in section 2.2 which deals with text representation of addresses. This provides three conventions for representing IPv6 addresses.
The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the hexadecimal values of the eight 16-bit pieces of the address. Examples: FEDC:BA98:7654:3210:FEDC:BA98:7654:3210 1080:0:0:0:8:800:200C:417A Note that it is not necessary to write the leading zeros in an individual field, but there must be at least one numeral in every field (except for the case described in 2.).
Due to some methods of allocating certain styles of IPv6 addresses, it will be common for addresses to contain long strings of zero bits. In order to make writing addresses containing zero bits easier a special syntax is available to compress the zeros. The use of "::" indicates multiple groups of 16-bits of zeros. The "::" can only appear once in an address. The "::" can also be used to compress the leading and/or trailing zeros in an address. For example the following addresses: 1080:0:0:0:8:800:200C:417A a unicast address FF01:0:0:0:0:0:0:101 a multicast address 0:0:0:0:0:0:0:1 the loopback address 0:0:0:0:0:0:0:0 the unspecified addresses may be represented as: 1080::8:800:200C:417A a unicast address FF01::101 a multicast address ::1 the loopback address :: the unspecified addresses
The third convention deals with mixed IPv4/IPv6 environments and I have rarely seen it used, so please check the RFC if you wanted more details.
What do this representation 1001::1 64 means?
Not 100% sure what the "64" part is doing here. While there are several things you could be intending, there are two cases that are most likely. Given the information above, I will answer both cases below.
Missing slash (i.e. representation is giving both the address and the network)
Space added accidentally (i.e. this was meant as a single address)