Queue depth is very complicated, and you need some specialist software or hardware to gain access to how commands are being queued by the operating system and applications.
If you open task manager you will notice that a typical system has hundreds of threads running. Most of those will only ever reside in system RAM, but some of those threads will require disk access.
An AHCI SATA controller supports native command queuing (NCQ). That simply means that several disk access commands can be queued and all sent at once. An HDD or SSD can only act on one command at a time. But if that SSD also supports NCQ then it can receive several access commands at the same time, rather than having to wait on each access command being sent, before it can then act on the next command. By doing this a lot of time can be saved, and therefore increased performance.
Queue depth is simply the amount of commands that can serviced at once. For a typical PC user, this will be quite low, in the region of a queue depth of between 1 and 3. However, most manufacturers claim that their SSD can handle upto a queue depth of 32, so we test at upto a queue depth of 32.