OKAY.

So the chord A major has in it the notes A C# and E. If played with A as the lowest note, C# higher and E highest in steps of a third then this is called 'root position'. The tonic is 'at the bottom' in such chords. Put simply, if you put these notes in a different order or use bigger steps you get different inversions and voicings of that chord. These affect the sound.

My pet 'thing' is when people play the bottom E string with a D chord; it often sounds 'wrong'.