Quality of Service (QoS) is a method to prioritize/penalize predefined types of traffic. QoS is a fairly complex topic so we'll cover only enough basic principles to help configure an initial setup.
Basically, QoS performs the following:

• Identifies traffic
• Classifies this traffic
• Marks packets
• Defines classes and subclasses
• Set class parameters (such as reserved bandwidth)

You might have heard the above points when describing “diffserv” (differential services). Diffserv is the only QoS type FreshTomato can perform.

It's often said that the QoS is a technique to mitigate lack of bandwidth when an upgrade is too costly or even impossible. More precisely, QoS also allows you to prioritize latency-sensitive traffic (such as voice) even when there's no network congestion. Thus, QoS introduces controls to allows you to protect certain traffic.

Here is some commonly used QoS terminology:

• Traffic: This is the stream of data between two nodes. This can be unidirectional (such as over UDP)
  or bidirectional (such as over TCP)
  
• Best effort: This is also referred to as “no QoS”. This works on the principle of “first come first served”,
  the default queuing mechanism applied to network interfaces (AKA “FIFO”). When QoS is operating, best effort usually means that class of traffic that handles “everything else”. It is therefore the lowest priority of all classes of traffic passed.
  
• Classification: This is the process of internally identifying packets based on predefined parameters.
  For example, “from IP”, “to port”, “protocol=tcp”.
  
• Marking: This is the action of actually colorize a packet (modify the Type of Service/DSCP field of the IP header)
  with a locally significant meaning. This will help matching packet simply looking at the ToS/DSCP header. This process it not always performed.
  
• Queuing: This Outbound only process is the process of buffering data that the network can't process in real time.
  There are many forms of Queuing in QoS. This is divided in classes.
  
• Policing: This is the [Inbound and Outbound] process of dropping packets in a controlled way.
  
• Class Discipline: This is the rule applied to all the classes of data in the priority class goes first no question asked, if priority is empty apply WFQ (weighted fair queuing) amongst all the other classes, etc. There are dozen of queuing disciplines but only few are supported by FT.
  

Looking at the list above let's put everything together:

Ever experienced a poor VoIP call from your LAN into Internet? Do you get media buffering too much? Is anyone in your LAN generating too much traffic? These are all things QoS can address for you. Please notice: QoS is NOT auto-regulating, you will need to define manually most of the parameters and tweak them as needed throughout time.

As shown in the above screenshot, QoS doesn't work when CTF is enabled. This is because CTF causes network packets to be routed in hardware, outside of firmware control or monitoring.