don't fret it just yet. Special relativity is probably among the most conceptually difficult material we will cover all semester, and it involves relatively subtle arguments and reasoning. (Oddly enough, the math is straightforward.)
So, if you felt a bit lost during today's lecture, don't worry, it is not at all unusual. It will take time for the arguments and really the
necessity of relativity to coalesce properly for you, it probably isn't going to happen just listening to me for 45 minutes.
Read through the notes (really read, don't just scan for highlighted items), read through your textbook, and don't hesitate to ask me if there is something you're not getting.
It will still be weird even when you think you understand it. The weird won't go away, but the confusion will. There is a difference - weird can still be logically consistent or even a necessity to have logical consistency.