It's actually really hard to explain
link It's similar to saying cute, 或者 at least cute is the closest word in English. It's usually referring to one specific trait ex. glasses.
If someone says someone in general is moe they usually mean that they're lovable, make 你 feel like taking care of them, not totally independent etc.
I'm not 100% sure (try asking someone who does think of this) but people might think Kanade is moe because 由 the end of the series 你 do want to take her in an hug etc. she's just one of those characters.
