computer networks – understanding formats of iPv6 adresses


I am trying to understand the difference between these two formats of iPv6 addresses:

2001:638:709:a::1/64

2001:638:709:a::/64

From what I understand, the 2001:638:709:a (in the second line) is a prefix and /64 is the length of bits it occupies. It would be a full ipV6 address if it includes 00000s instead of ::/. But then what’s the first line representing? What’s the 1 showing before 64?