You don't really need a mat file. You need ground truth data for training. So, you need a set of images with bottles in them and bounding boxes for the bottles in the images.
The mat file just contains an array of structs specifying the name of the image file and the bounding box of the bottle in that image.
That line of code loads a variable named stopSigns (and a variable named layers) from a MAT-file named rcnnStopSigns.mat. The variable is stored inside the MAT-file.
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