What does one and two ticks mean on whatsapp when using an iphone?

I've sent a message fom my android phone to someone who is using an iphone4 via whatsapp. I either get 2 ticks, 1 tick or a little clok. This time i just have 1 tick. Does this mean there's a notification from whatsapp on their phone but they haven't opened it yet? Confused: s

The clock means that your message hasn't been sent to the other person
1 tick means that they haven't received the message
2 ticks mean you have sent the message and their phone has received it, but it doesn't necessarily mean that they have seen it.
Hope this helps!

One tick means it's been sent from your phone, two ticks means its been delivered to theirs. It might stay on one tick if their phone is off or has no signal.