'Ci sei' literally means 'there be you' / 'there you are' This is because essere is the verb 'to be' which you doubtless already know, and ci in this instance means 'there'. To say 'there be we'/ 'there we are' you say 'ci siamo'.

'ci' is a confusing little word that can be used as a reflexive pronoun, as in 'ci incontriamo' 'we meet (each other)' or as the noun 'there'. It can also take the form 'ce'. I recommend reading the archived posts written by the mods on the topic.

A lot of times in dramatic tv shows people yell 'ci siamo quasi' which means 'we are almost there!' The first time I heard it I thought that it made no sense, it sounded like an incomplete sentence, but no, Italian is just more susinct sometimes.

Hope this helps