A measurement result is considered accurate if it is judged to be close to the true value.
e.g. Bob, Bill and Becky are asked to determine the capacity of a box for storing ball bearings. Each of them uses a different ruler and only one takes care doing it. Here are their results:
The true value of the capacity is 545 cm³, so Becky's measurements are the most accurate. Her calculated capacity is the most accurate.
You may also need to indicate how accurate an answer is (called uncertainty).
e.g. the speed of a wave on a string is 34.5 m/s within 0.5 m/s.
We would often write this as 34.5 ± 0.5 m/s