A pixel may refer to either a single dot on a screen (resolutions are often represented by pixels) or a specific type of HTML code.
For the first definition, in a digital advertising scope, it usually references the size of a creative. These sizes are usually represented as width by height. A 300×250 ad creative simply means that it is 300 pixels wide by 250 pixels high. Other common sizes include 728×90 and 320×50, but there are endless possibilities.
For the second definition, it most commonly serves one of two purposes. First, it can be used to retarget advertising. It can also be used to track website conversions, including form completions and ecommerce.