Something a girl/guy will write to you, if she's/he's either really excited to talk to you, or really wants to chat with you. (if they write more i's then you know it already ;)
P.s. if they write hii, then they're just happy to text you.
Alexa: Hiii
Jake: Hey how are you?
Alexa: I'm great, so happy to talk to you!
...
When a girl says hiii, shes usually either really excited about something, or really wants to talk to you. Or if your talkingtechnical terms, she is saying it in a dirty way and really wants to have sex.