You are welcome. Don’t worry, as you continue to read and explore your knowledge base will steadily expand and deepen. Curiosity and motivation will take you far. Vacuum tubes are actually pretty basic devices but there are so many variations and applications with regard to function and utilization.
They have stood the test of time for very good reasons. When used properly in audio components they’re capable of exquisite and authentic music reproduction.
I am not an electrical engineer and @atmasphere (Ralph) can provide a much better explanation. Essentially linearity refers to the degree of disruption/deviation/distortion of an applied audio or electrical signal. The closer the output signal resembles the input signal (Less deviation) the more linear or pure a device is said to be.
This in part explains why DHT type tubes can be used without the need for applied negative feedback (NFB) compared to indirectly heated tetrode/pentodes generally speaking. No doubt other factors are in play.
You will often see DHT SET amplifiers with zero NFB. There are pros and cons to everything. It all depends on what is sonically being sought.
Charles