Megapixel is the (approximate/rounded) amount of pixels used to capture the image.
5000*5000 pixel in a square would be 25 megapixel.
One pixel is (simplified) a single dot with a certain colour/brightness.
The more dots you use, the sharper you can make the image.
But: a lot of other factors also determine the quality of the image. Everything above 5-15 megapixel is more than sharp enough to make images that look good. More megapixel only help if you enlarge a tiny portion of that image or want to put the photo on poster size.
The most important quality is size – size of the individual pixels and size of the lens – as a bigger lens will simply let more light into the camera, which makes it easier to get the accurate color.
So DSLR will have a better quality in almost all situations. (If you want to enlarge a specific portion of the picture, with a DSLR you can attach the correct lens, and the lens will enlarge it.
Current smartphones show a incredible quality though, and are up to the task for many situations.
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