Moving averages are used to smoothen out spikes on certain days (caused by backlogs clearance etc).
Hence the 7 DMA graph gives a more clear picture of the new cases for COVID-19 and understanding the spike and the fall.
A moving average means that it takes the past days of numbers, takes the average of those days, and plots it on the graph. For a 7-day moving average, it takes the last 7 days, adds them up, and divides it by 7.
For something like new cases of COVID, you can see why this can be handy: it provides you with an average line over time, and it knocks out these big peaks and valleys to the average over a period of time.
This is just a simple 7 DMA and there could be better ways to showcase the same.
Thanks to Rajendra Pai.