It simply means to do something like try a single connection attempt to a single port with a lot of time between.
But what "slow" means and how much time is not defined. 1/hr? 1/day? It depends on how stealthy the person wants to be and what their goals are.
nmap has many config options to do this (read the manual: https://nmap.org/book/man-performance.html)
T0 sends a single connection attempt to one port at a time with 5 minutes in between. There are numerous finer config options where you can define exactly what you want to do.
I challenge the statement that noisy network scans are not performed by advanced hackers. It really depends on the situation. There is no reason to go slow on a public IP. Internal IPs, once you are inside and re-enumerating, would need this, but this is not something that only advanced attackers would do. It's just a response to the reality that noisy scans would be discovered and so would the attacker. Typically, the attacker would listen to traffic or do other enumerations to get a sense of what servers are available and what they serve.