Here’s a fascinating stop-frame animated YouTube from Vi Hart talking about how musical notes are made up of different frequencies. It uses an unfretted instrument (viola) to show how you can work out where along the fingerboard the different notes appear, so you can see how this translates to the uke & why the frets are where they are.
Towards the end, it also explains how our ears are not only really good at distinguishing between lots of different sounds occurring at the same time & working out that you can hear a piece of music, car horn & your mobile ring tone all at once, but ‘fills in the gaps’ where some of the frequencies are missing. So that’s why we sometimes say we can play chords & leave out certain notes & it doesn’t matter (as the ears compensate) or that it can be pleasant when people in the group play slightly different inversions of the chords, to complete the full range of what the ears expect to hear.
Enjoy watching & then maybe follow it up with looking at our Very, Very, Very Basic Understanding of Music for Absolute Beginners & other Weekend Workout or music theory posts. Hopefully that will help make things hang together more easily if you’re still fairly new to the theory side or find it a little daunting.