You mean as in 720p? P stands for Progressive scan or non interlaced. Back in the days of analog TV with CRT tubes and mechanical tuners there was interlaced scanning. TVs used to have an electron beam that swept across inside the tube spraying electrons on the back side of the viewing screen. The sweeping was not fast enough at that time to spray every line so it would spray the odd lines one sweep across/down and then the next time it sprayed the even lines and the coating on the tube would have the light persist long enough that it looked like the whole screen was lit up. When computers came along they slowly got faster and faster scan rates and were able to sweep the whole screen in one pass so there was no interlacing (480i). Computer monitors and TVs when they went to higher resolutions went back to interlacing or scanning half the lines in a pass again but over time sweeping got faster and faster and interlacing was less and less common. Today's TVs are digital and don't use electron scanning (spraying) do not have a sweep but instead use LCD displays with back lighting and the LCDs are drived directly each pixel set is lit up a the same time so everything is "p" now. There still is content and hardware out there that are interlaced but it is obsolete now. For all practical purposes all streaming is going to be non interlaced on smart TVs..... maybe online some really old content could be interlaced but likely someone would either go back to original source and redigitize them to progressive scan.
I have a 1080i DVD changer, that is it upscales from 480p to 1080i. Broadcast (over the air) TV has interlacing on 480i and 1080i resolutions as it takes less bandwidth. I think with ATSC3.0 we may see the "i" modes be less and less popular but it is possible that streaming may all but kill interest in broadcast TV. I used to watch broadcast TV mostly but cheap high speed internet and streaming for free has replaced it in use primarily because the stations that once came in good many no longer do, I'm thinking that they dropped their power output of their transmitter to save money and maybe new construction is blocking the signal perhaps.