Fundamental Matrix Accuracy

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I am working on 3d Reconstruction of Scene. I matched the feature of 2 images. I have the Keypoints1 and Keypoints2. I have Fundamental F and Essential Matrix E. Now I have to check X'FX =0. I know X' is the coordinate of second image and X is the coordinate of image of the first image. I have few questions can anyone please help me.

Is X' and X are the keypoints1 and keypoints2 ? Is it (x, y) coordinate of that keypoints? The product of X'FX, this will be in Mat format, if I not wrong. How this will be equal to 0? Please can anyone help, sorry if my question is silly.

Thanks in advance.