What does tu no estas segura de ese mean?

You don't know is the English equivalent of 'Tu sais pas'. In the Word by Word translation, the personal pronoun 'tu' means 'you'. The adverb '[ne]...pas' means 'not'. The verb 'sais' means '[you] are knowing, do know, know'.

Very often, this is the beginning of a conversation and the equivalent in English would be something like - you'll never guess, but.... or guess what?