It's a more emphatic way of saying "probably" than the more neutral "tabun" (たぶん).
Here are some examples:
* "Kitto ame ga furu" (きっと雨が降る) - "It will probably rain."
* "Kitto kare wa shiritai" (きっと彼は知りたい) - "He probably wants to know."
* "Kitto watashi wa okureru" (きっと私は遅れる) - "I'll probably be late."
"Kitto" conveys a stronger sense of confidence and likelihood than "tabun". It's often used when the speaker has a strong feeling about something or when they are confident in their prediction.