> There was a post here recently that if you bought pretty much any stock last year, you wouldd profit, so I'm not sure what this is?
It was hard to figure out the real numbers of profit you can have with a single trade and multiple trades on one stock within a year. So many reports say "AMD was the best stock of the year 2019", but when you look at the report it looks like they used a different metric (maybe percentage). So it is just a try to give people a better overview about the numbers.
> Is this just if you bought one share on 1st Jan and sold on 31st Dec?
The anchor link shows the maximum profit, meaning you can buy and sell as many times as possible between 1st Jan and 31st Dec. In the section above you find the analysis for the metric as you said.