The capture area is the area of sunlight that you'll be capturing. If you just sit a black jar out in the sunlight then the area of the jar exposed to the sunlight is the capture area. If you have a box with a window then the window is the area of sunlight you'll capture and no more. If you have reflectors then it gets more complicated.
Note that sunlight doesn't always come directly from the sun. I also reflects off of clouds, snow, water and other reflective surfaces. For most solar cookers this won't change the area of the hole, the capture area, that the sunlight has to enter through. An exception to this would be a standalone black jar when light can hit it from all sides, though unless the reflective surface is something highly reflective like snow or water then it won't make much difference.
Capture area with reflectors
Often people think the capture area is the surface area of the reflectors, but it isn't. To bring this point home, the diagram below shows a ridiculously long reflector on the left along with its capture area. It should be clear that the shape of the walls of the reflectors don't affect the amount of sunlight captured compared to the two reflectors to the right of it.
Note that a badly designed reflector, possibly combined with other reflecting surfaces, can reduce the benefit of a large capture area by reflecting the sunlight right back out of the solar cooker. The following photo shows an early stage of the design of my cone solar cooker, illustrating that with large rectangular reflectors a lot of the sunlight would have been lost. Shortly after realizing this I designed a cone for it instead.
Finding the capture area using a shadow
If your cooker doesn't have any reflectors on it but instead consists of just a window on a box then the capture area is the area of the window (see rightmost cooker in the diagram of different capture areas above.)
But if your cooker has reflectors then point the solar cooker directly at the sun, like you would want to when cooking (see the diagram below.) Put a flat surface behind the cooker for the shadow of the cooker to fall on. This surface must be facing the sun directly, like your cooker is. The shadow made by the reflectors on the surface is the capture area. You can make measurements of the shadow for working out the capture area.
Capture area calculation example
When I built my cone solar cooker, at one point during the design I had to figure out if the cone would be adding a significant benefit to the cooker. The cooker already had a large reflective bowl and so a cone reflector may not have been needed.
Step 1. Calculate the capture area of just the bowl. The capture area for the bowl is a circle and the formula for the area of a circle is the radius of the circle multiplied by itself multiplied by pi (pi is always 3.14). So you'll need to measure the radius of the bowl (the radius is the distance from the center of a circle to the edge of the circle as shown below.)
Step 2. Calculate the capture area of the cone. The capture area for the cone is the circle made by the top of the cone. Once again, the formula for the area of a circle is the radius of the circle multiplied by itself multiplied by pi (pi is always 3.14).
Step 3. Calculate how much bigger the cone capture area is than the
bowl capture area:
I figured that 2.4 times the capture area would be worth it so I went ahead and made the cone.