Most camera's you can buy these days have one of many different sensor sizes. The sensor creates the specific aspect ratio of your images that is usually 2:3 and are made up of a specific number of pixels (megapixels) and that changes from camera to camera.
However, there has always been a standard sensor size that equates to 35mm diagonally across and also measures up to a 2:3 aspect ratio. This sensor size is what we refer to as a full-frame sensor which was made a standard across the board of cameras from the time when film was still commonly used.
There are no major issues to having a smaller sized sensor than that of a full-frame 35mm, but there are some notable differences that you should be aware of and that has got to do with the focal length of the lenses that you use. If you were to shoot on a camera that has an APS-C sensor size that is 1.6x smaller than a 35mm full-frame, then your lenses would deliver a field of view that is 1.6x greater than the same lens on a full frame. For example, if you were to use a 50mm lens on a camera that has a 1.6x crop factor, then you will get the equivalent focal length and field of view of an 80mm lens on a full-frame 35mm camera (50mm x 1.6 = 80mm).
Take a look at the video below by the YouTube channel: ProAV TV on Explaining Crop Factors...