Frames are generally fine to be left alone and it’ll do the right thing. Playing with them is a more expert-type thing and probably requires more explanation than just diving in - so well done for experimenting 
I’ll see what we can do to explain them in more detail. Would some use-cases help? Or…?
Regarding note input, using the spacebar doesn’t really enter rests, it just moves the caret along by the length of the note you have selected in the left panel - similar to the right-arrow, although that uses the grid length. In your example above, using the right-arrow to get to the last 16th of the bar and then entering a note would have been easier / quicker?
FYI - shortcut for changing the grid is Alt + [ or ] (square brackets).