I found this video to be an extremely helpful and concise explanation of music theory basics:
https://youtu.be/rgaTLrZGlk0"Learn music theory in half an hour" is obviously an exaggeration, but it really comes astoundingly close to fulfilling that promise. It contains a lot of information and each part builds on the previous parts, so it requires focus and maybe a few repetitions to 'get it', but I think the approach is fantastic for showing how many ideas of music theory are deeply connected.
Not a book as you requested, but hopefully you'll find the other reply useful. As for some of your specific questions:
Frequency is the same concept as radio frequency, but in this case refers to something we can directly sense. Radios transmit electromagnetic waves, which are photons moving at a certain rate, measured in Hertz, or cycles per second. Sound frequency refers to movement of air waves, so a more accurate analogy than radio waves is waves in a pond when a rock is thrown in. Human ears are sensitive to frequencies between 20 Hz and 20,000 Hz, so any sound you hear is a combination of frequencies. Natural language is helpful here, since higher frequencies sound 'higher' and lower frequencies sound 'lower'.
A tone is a sound at a specific frequency, also known as a note. For example, 440 Hz is designated as the note A4 by the Geneva conventions, and this is what most instrument tunings are based off.
Twelve tone equal temperament is the tuning system nearly all modern Western music uses. Certain ratios of frequencies sound pleasant, especially ratios with small numbers, such as 1:2, 2:3, and 3:4. So if we know 440 Hz is a note in the system, it would be nice to also have 587 Hz, 660 Hz, and 880 Hz. However, these frequencies will only really sound good when played with that original 440 Hz, not necessarily with each other. So instead of using them exactly, we approximate them in a useful way. The 1:2 ratio, the octave, is generally considered to be the most important, so that ratio is kept, but otherwise the notes are equally spaced (human hearing is logarithmic), or equally tempered. The most popular tuning system has twelve tones. There's no note at 660 Hz, but there's one at 659 Hz, which is pretty close, and there happens to be one at 587 Hz. Other ratios are also represented reasonably well.
A key is a collection of notes that sound good together, based on the ratios of their frequencies. The alternative would be chromatic composition, where all 12 notes are used and none is obviously 'more important'. Most music is in a specific key, but uses some chromatic notes to make the melody more interesting.