Estimate is approximating to something that is easy to calculate in your head.
3/4 and 4/5 are each close to 1, so the answer must be less than 2. Also, 3/4 and 4/5 are bigger than 1/2 so the answer must be bigger than 1. So far, we can estimate the sum is between 1 and 2. We could go further if we realise that 4/5 is bigger than 3/4. So the sum is bigger than twice 3/4 which is 1 1/2. Now the estimate is that the sum is between 1 1/2 and 2. The more we can visualise and compare the numbers the better the estimate we will arrive at.