Setting a date explicitly will cause the task to move and a constraint to be set.
If you explicitly set a start date your task will move to the date you selected and a "start no earlier than" constraint will be set. If you explicitly set the end date your task will move and a "finish no earlier than" constraint will be set - If both these constraints are set, this can lead to an issue whereby changing the end date of a task does not update it's duration.
If you want to set the duration and avoid setting constraints against tasks you should enter the duration value in the duration field, or you can click and drag the right end of the task bar within the chart. Either of these approaches will adjust the task duration leaving the start date unmodified. The end date will calculate based on the start date + new duration.
If constraints are new to you or you're not sure what they do, taking a few minutes to read the help topic on the subject which includes a step-by-step example would be recommended.
To open the help, run MindGenius and double-press the "F1" key, then search for "constraint".