1080p and 1080i are the same resolution which is 1920 x 1080 however they are different in how they are broadcast.
In 1080p the p stands for progressive which means the video is drawn line after line. 1080i i means interlaced, the video is broadcast as 30fps but plays back at 60fps. The frames get shown twice because the video is interlaced horizontally by two 1920 x 540 parts of the footage.
This can be best seen by his graphic:
On the left you can see the horizontal lines made by the interlacing process. As the image gets repeated to double the frame rate it doesn’t look very good or natural. Progressive on the right looks the cleaner image (footage).
Now days it’s mostly TV (satellite) that uses interlaced footage, From Videographers to online streaming is done in progressive. For sports it would be better to watch a high bitrate 720p output than a 1080i one. Whilst 1080i can be labeled as 60fps it’s not a true 60fps, rather repeating.