The ‘p’ and ‘i’ after the resolution, like 1080p or 720i for example, refer to the way in which each frame of the footage is displayed. The ‘p’ stands for ‘progressive scan’ and ‘i’ for interlaced scan.
In a progressive scan, the frames are displayed one row at a time, from right to left. It presents the complete frame or image all at once in an uninterrupted and seamless flow and is used on modern devices.
In an interlaced scan, the frames are split into odd rows and even rows, and the two are switched (or alternated) really fast so that you get a complete image. It was once the preferred method of display for older TVs, but, in modern devices, it causes flickering or weird lines, which is more pronounced in fast-moving scenes.