Can someone please explain

brighter = sharper-sounding midrange and/or treble and tight bass
darker = like brighter above, but more accurate
warmer = softer-sounding midrange/treble and more bass

Neutral sound means more accurate. Ideally, you don’t want to boost any particular frequency. The music should come together as one cohesive whole and no frequency should annoyingly stand out.

You can learn by looking at the waveforms of the track you’re listening to. Try to make some EQ changes using something like Wavepad and see where that goes.

Some tracks sound like "recordings" while others sound like organic music. It all depends on many factors...but the recording quality itself is of the utmost importance (using the right microphones).

Look up amplifier classes to learn more. SS or tubes? that's a personal preference. Good luck.

All of these terms are just "poetic license"! I'd rather look at frequency and pulse response, polar dispersion and power handling. Speakers that measure better will sound better. Unlike electronics speakers haven't evolved  to the point where their flaws are hidden from measurements!

These terms are listener oriented audio terms. While waveforms / theories are interesting if you are an engineer they are not of much actual value to a audio enthusiast. These terms and others are important in comparing musical output from a system or component.

This may be a bit overwhelming but here is a glossary from Stereophile.

https://www.stereophile.com/content/sounds-audio-glossary-glossary

I recommend reading Stereophile And The Absolute Sound for reviews to get a basic understanding. Specific questions can be answered on the forums... but you can get a lot of “religious zealots” and biased opinions, some well meaning, some not. While these publications have specifications associated with equipment the bulk of the content and value is about listening comparisons.
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Inexpensive equipment frequently is bright... with a high preponderance of treble, which over time you may realize is actually distortion. So, bright may be very detailed sounding... and you may find it fatiguing. Dark can mean an absence of treble.
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But they are relative terms. So you need a ruler against which to judge. If you have equipment find every review and comment on line about it so you can understand how it is described. If you do not, or it is too low end. Go to a high end audio store. Tell them what kind of music you like and find some equipment you like. Write it down. Then read all about it from sources like above or Tone Magazine, positive feedback. Try and get to know a well reviewed brand. Audio Research, MacIntosh, B&W, NAD,.. if your find this kind of research interesting, this may be a good hobby for you. It is very complex, and specifications are useful for really gross decisions on the sound you want. This is about listening and talking about overall tonal balance, slam, sound stage, and rhythm & pace... the musicality... does it emotionally connect with you. 
A good manor to learn and acquire is to find some flagship products who’s sound you really like and the work down their product ines for something you can afford. Most high-end companies have a house sound which applies to all products. So you can move up their product lines over time.

GOOD LUCK.