What does it mean to be "in suspense"?

Solution 1:

Suspense (the correct spelling) is an emotional state. Just as one can be in fear, one can be in suspense.

Solution 2:

I think understanding the origin of the word is helpful in picturing its meaning in this case.

Etymonline relates it (through its French origin) to suspend. If you imagine a person in suspense to be suspended in the defer or delay sense, or metaphorically hung up in the hang sense, in their desire for knowledge, you can get a good feel of the mental state suspense is describing.

Further, ODO states the French origin as suspens 'abeyance'. Abeyance simply means a state of temporary disuse, which has similar connotations.

You can also think of suspense being related to curiosity. When you are intensely curious about something but the situation has temporarily suspended satiation of that curiosity, you experience suspense.