Why NFA is called Non-deterministic?
"Deterministic" means "if you put the system in the same situation twice, it is guaranteed to make the same choice both times".

"Non-deterministic" means "not deterministic", or in other words, "if you put the system in the same situation twice, it might or might not make the same choice both times".

A non-deterministic finite automaton (NFA) can have multiple transitions out of a state.  This means there are multiple options for what it could do in that situation.  It is not forced to always choose the same one; on one input, it might choose the first transition, and on another input it might choose the same transition.

Here you can think of "situation" as "what state the NFA is in, together with what symbol is being read next from the input".  Even when both of those are the same, a NFA still might have multiple matching transitions that can be taken out of that state, and it can choose arbitrarily which one to take.  In contrast, a DFA only has one matching transition that can be taken in that situation, so it has no choice -- it will always follow the same transition whenever it is in that situation.