In more detail
Each pixel on an LCD or OLED panel is made of red, green and blue subpixels. A dead pixel has all three permanently off, so it shows as a black dot on a white screen; a stuck pixel has one or more subpixels permanently on, so it shows as a red, green, blue or other bright dot on black. Cycling a screen through full-colour fields (black, white, red, green, blue) makes either kind easy to spot. A "hot pixel" in photos is a different fault, on the camera sensor rather than the display.
Why it matters
Check a new monitor, laptop or phone as soon as it arrives, while the return window is open. Manufacturers' pixel policies often tolerate a few defects before replacing a panel, and they may treat bright dots more strictly than dark ones or count only defects near the centre. Stuck pixels can sometimes be revived by rapid colour flashing or gentle pressure; dead ones are hardware faults.
Example
A 4K panel has 3840 × 2160 = 8,294,400 pixels, or 24,883,200 subpixels. One faulty pixel is about 0.000012% of the screen, yet a single bright dot on a dark background is easy to see from normal viewing distance.