My QoS Primer for Small Business Management
Sam C. Chan  Feb 27, 2017

Digital communication is conducted by transmitting packets.

At the ISP & backbone level, packet prioritization is enforced at all paths, according to their policies, objectives and obligations. Some traffic receive preferential treatment, while others are delayed, or even lost/rejected. They regulate the rate (speed), quota (amount), and type of traffic down to the most minute detail.

At the customer's premises, things are uncontrolled: no limits, nothing discriminated, nothing blocked, pure FIFO (first in, first out) --unless IT measures and apparatuses are in-place to enforce policies.

A typical Internet subscriber line is 90%~99% idle on average, just like a typical workstation CPU is 99% idle over a 24-hour period. Capacity allocation must consider the peak demand. The point here is: bottleneck & contention, during those brief but critical moments.

QoS (Quality of Service) is the computer science term for prioritizing packets, aka Traffic Shaping. It is done by reserving, say, 10% of the available bandwidth, and give it to designated priority packets, as a boost, thus preventing critical applications (RDP, VPN business apps) with light but urgent traffic from being starved by bulky auxiliary applications (downloading, streaming, email attachments, etc.)

The result is a slight reduction in overall utilization efficiency (more wasteful), because the line is never allowed to saturate. Such added delay might not even be discernible by those affected. On the other hand, the few favored applications would see a very noticeable boost in responsiveness.

Within the gigabit LAN, bandwidth is abundant. Meaningful contention is practically a non-occurrence. WAN typically have outbound bandwidth of 0.5~3.6 Mbps, for 99% of small businesses in the US. That's only 1/2000th ~ 1/300th of LAN speed. Congestion and show-stopping degradation is certain recurrence, begging for active control.

QoS implementation can range from basic to extremely sophisticated and complex. It is not a miracle solution, as there are scenarios where prioritization won't resolve issues, in which case it needs be implemented in conjunction with other techniques. For most small businesses, lack of QoS is without-a-doubt the top bandwidth-related issue, and it's considered mandatory.

NOTE: This has been thru several revisions in the past 10 days. I narrowed it down to absolutely essential points. Each point is condensed into a single compound sentence.
Such technical concepts must be conveyed in a well-structured manner, in strategic sequence. You need to reread it sevreal times. Once you grasp the big picture,  you're able to ask specific questions, and comprehend the responses.