“Moe” refers to the emotional attachment one can feel for fictional characters. It’s use is popular in otaku culture.

Unlike the definition you get if you google the word, the character does not need to be female, vulnerable, innocent, or even young. Moe refers to the feeling experienced by the viewer, not a character type.

You might design a character to elicit moe, but there is no single way to do that. Different people are into different types of characters.