A word to describe responses such as "I see" and "Okay"
These phrases and words in this context are acknowledgements, indicating recognition.
I think phatic is the word. A phatic expression is one whose only function is to perform a social task, as opposed to conveying information. Utterances such as hello, how are you? and nice morning, isn't it? are also typically phatic.
https://en.wikipedia.org/wiki/Phatic_expression