Estimated effort is the amount of work (e.g. in hours) it takes to get the job done. Value defines how valuable the outcome of the task is (which should correlate with how quickly you actually do the task).

So given two tasks: Task A with 5 hrs of effort and value of 100, and Task B with 5 hrs of effort and value of 20, you should first do Task A since it produces more value. Value figures should be relative so when you estimate how valuable some task is, compare it to at least two other tasks.