To form an opinion or give an answer about something when you do not know much or anything about it. I had to guess the answer to the last question on the test.
To estimate or suppose something without sufficient information to be sure of being correct. She guessed the total cost of the groceries before reaching the checkout.
To predict or conjecture something based on intuition or limited information. He guessed that it would rain later in the afternoon.
To think or suppose; to believe something is true, often with some uncertainty (chiefly U.S., informal). I guess you're right about the meeting being cancelled.