If you're referring to the limits discussed in integration and volume of revolution, it's nothing that big.
Integration is used to calculate the area below a curve in a certain region that is defined by the x-coordinates or y-coordinates.
These are the limits. It is only between that region that the area is required. Beyond that isn't. Eg. The curve cuts the x-axis at 3 and at the line x=7 and you are asked to find the area below the curve (usually shaded and shown) the limits you will use are 7 and 3. (3 to 7).
Once you've integrated the function, you slip these values in place of x and subtract them to get the area.
Similarly this also work to calculate the volume of revolution.