The words "it's okay" and "that's great" are both expressions of approval, but they have slightly different meanings.
It's okay is used to express that something is acceptable or satisfactory. It can be used in a variety of contexts, but it is often used to express that someone is not upset or angry about something. For example, if someone asks you if you are okay after you have been sick, you might say "it's okay" to mean that you are not feeling too bad.
That's great is used to express that something is good or excellent. It is often used to express excitement or enthusiasm about something. For example, if you hear that your friend has gotten a new job, you might say "that's great" to express your happiness for them.
In the example sentence, the speaker is asking the other person if they can go somewhere. If the other person says "it's okay," the speaker understands that the other person is willing to go, but they are not necessarily excited about it. If the other person says "that's great," the speaker understands that the other person is excited about going.
Here is a summary of the differences between "it's ok" and "that's great":
Expression - Meaning
It's ok - Something is acceptable or satisfactory. (Example: Person commenting on the quality of the hotel they are staying at. Person 1: "How do you like the hotel you are staying at?" Person 2: "It's OK." Meaning on a rating scale of 1 to 5, where 5 is the best, the hotel would receive a rating of 3.
That's great - Something is good or excellent. (Example: Son, "Mom, I passed my exam." Mother: "That's great."
Context - Used in a variety of contexts.