In the profile view in an approach plate, there is a visibility number next to the MDA or DA. What does it mean? Is it limitation to use the approach?
If this question is about the US, the FAA's Chart User's Guide is a useful source for questions about approach plates or any other charts. It says:
This section gives the pilot the lowest altitude and visibility requirements for the approach. There are two types of landing minimums: Straight-in landing or Circling. Straight-in landing minimums are the MDA and visibility, or DH and visibility, required for a straight-in landing on a specified runway. Circling minimums are the MDA and visibility required for the circle-to-land maneuver.
The visibility values are shown after the DA/DH or MDA. They are provided in statue [sic] miles or runway visual range (RVR). RVR is reported in hundreds of feet. If the visibility is in statute miles, there is an altitude number, hyphen, whole or fractional number, e.g. 530-1. This indicates 530 feet MSL and 1 statute mile of visibility. The RVR value is separated from the minimum altitude with a slash, e.g., 1540/24. This indicates 1540 feet MSL and RVR of 2400 feet.
The number is indeed a limitation: if the required visibility doesn't exist then the pilot can't use the approach. However, there are some differences in what that really means depending on the type of flight: an airline flight (part 121) can't even start an approach if the reported visibility is less than the approach requirements; a private flight (part 91) can start an approach but may not land if the flight visibility is below minimums. This question might be relevant.
That is typically a Runway Visibility Range (RVR) number expressing the runway visibility in thousands of feet. The pilot will have to determine whether local weather conditions permit runway visibility at or greater than that published figure in order to successfully terminate the approach in a landing. If it cannot be done, the flight crew must initiate a missed approach.