In the output for netstat what does the [::] mean?

Well, more specificially, it's equivalent to the IPv4 "0.0.0.0", which, when LISTEN is specified, means "any IP address." You might also see "::1" which is the IPv6 loopback address.

In IPv6, a colon separates every 16 bits, or 4 hex digits. However, consecutive zeroes in the address can be "collapsed" or omitted one time in any IPv6 address. Even if the address is all zeroes. So, for example, 2001:0000:0000:0000:0000:0000:0000:0001 can be shortened down to just 2001::1.

So, evidently you have ssh listening and accepting connections from all IPv6 addresses.

That it listens on an ipv6 interface.