Full frame cameras are those whose sensor size is designed to be like a 35mm film camera. Film cameras had an opening of 24x36mm, and that was it. A full frame camera has a sensor size of about 24x36mm.
Crop sensor cameras have smaller sensors. There is a ratio – called a crop factor – that varies among camera manufacturers, of the sensor to the “full frame” size. For Nikon, it is 1.5; for Canon, either 1.3 or 1.6. Pentax is 1.5.
Here is a link to a good article: https://digital-photography-school.com/full-frame-sensor-vs-crop-sensor-which-is-right-for-you/
Generally, full frame cameras will give you a better image (less noise) at a given ISO, so you can push a little harder in low light conditions without worrying about noise. And they generally have better image quality in general.
But…you need to take the crop factor into account when comparing photos.
You need to multiply the crop factor by the focal length to get an equivalent focal length using crop factor or full frame (in terms of angle of view). That is, a photo taken with my Pentax (crop factor 1.5) using 50mm lens will look like the same photo taken with a full frame camera using a 75mm lens. Or my Pentax, using a 35 mm lens, will be like using a 52.5 mm (roughly a 50mm) lens on a full frame camera.
But wait – there’s more!
To really make an accurate comparison, you must also multiply the aperture by the crop factor. That means that a 50mm lens on a camera with a crop factor of 1.5 at an f/stop of 4.0 would be like a 75mm lens on a full frame camera at an aperture of 6.0. What does that mean? Well, all else being equal, the aperture of 6 is going to let in less light at any given shutter speed than an aperture of 4. That means that you must either use a slower shutter speed or a higher ISO, or both, to get the same exposure.
Here is an excellent video explaining all of this, with illustrative photos.