Active/Passive Hitsounding aka Clicking/Gameplay Rhythm
Active Hitsounding refers to hitsounds that you actually take an active part in (-> click), therefore, this refers to circles and sliderstarts; active hitsounds generate pressure behind the notes played and therefore should predominantly be set on already existing beats (or extremely natural-feeling additive mapping, but that's a rare exception).
Passive Hitsounding refers to hitsounds that occur without you actively participating (-> hitting) on them, such as sliderticks, reverse arrows, slider- or spinnerends; these do not generate pressure in playing and therefore should be feeling natural when silenced to emphasize longer/stronger notes in the music.
Take this example: You see, we have two sliders, 1/2 each - this will just sound like 4*1/2 hitnormal but effectively, it's played as 2*1/1, so you have to consider, when does it fit? Given that you only click the starts of the sliders, it could fit when the white ticks are (significantly) stronger than the red ones, resulting in a 1-(2)-1-(2) rhythm (1 = active, strong beat, (2) = passive, weak beat/no beat).
To visualize it better, you may draw a line under every active hitsound and leave the passive ones empty:
tl;dr: An active hitsound is when you click and there comes a sound, a passive one is when the object just makes a sound "by itself"/without an impact (-> click) required - keep this in mind, because passive are effectively weaker than active ones even when they "sound the same".