In general, this term means that the building must be set back from the road in front of it. You cannot build a building in the setback area, nor can you construct a gate or fence, or use the area for storage or parking.
If there is an indication regarding a setback, such as "setback required," be sure to check whether or not the land is in compliance with the "road access obligation" stipulated in the relevant laws and regulations. If "setback required" is indicated in the property summary or in the real estate advertisement on the portal site, the buyer of the property will have to build on a smaller lot than the current one when rebuilding in the future.
On the other hand, if there is a setback provision on a site whose floor area ratio is reduced by the width of the front road, it may be possible to consider the line after the setback as the front road boundary line, and calculate the floor area ratio based on the value of the road width when it becomes wider.