First, a few warnings (so that you know what to do if you decide to use in the future the series in the graph you have shared):
The series in the graph is actually called "Nikkei 225 Total Return Index", not "Nikkei 225 Total Return(s)". This distinction is important and I will tell more about it later on.
The graph of Nikkei 225 Total Return Index you have shared from its "General" page on investing.com, purporting to show the values of the index from early July 2019 through early July 2020, shows incorrect values towards the end (not your fault; investing.com has screwed up and it still does as of yet as far as that chart is concerned). Here is the correct one for your convenience (mine looks slightly different also in detail because the graph on investing.com appears to be on a weekly basis while mine is on a daily basis).
How do you read a Total Returns chart?
First of all, the chart you have shared does not show the (percentage) returns or something similar. It just shows an index ("Nikkei 225 Total Return Index") whose daily values are the (simple) arithmetic averages of the corresponding daily closing prices of the stocks in the Nikkei 225 index. These averages, however, are rebased from an initial value of 6569.47 as of December 28, 1979 and adjusted for certain events. This value, for technical reasons, is the same as the (main) Nikkei 225 index's value as of the same date. The adjustments are due to corporate actions, such as stock splits and dividends, and the stocks added to or dropped from the Nikkei 225 index since then. You can learn more about Nikkei 225 Total Return Index from its official web page if you like.
Nearly all most-quoted versions of popular stock indices, including Nikkei 225, are the versions called "price indices" and there are usually many other versions such as "net return" or "total return”. These are created to better cater to different needs of the investors. The main difference, for example, between these three is usually based on how dividends are accounted for. In a "price" index, the index values are not adjusted for most dividends whereas in a "net return" or "total return" index, the index values are adjusted by assuming any dividends distributed by the stocks in the index are reinvested (net of or with no taxes) back into all of the index stocks. Calculating the performance of a stock index using its "net return" or "total return" version rather than the "price" version is considered to be more realistic, especially over the long term.
Now then, I think it would be better to rephrase your question as "How do you read/use a Total Return Index chart?".
Well, the answer is like any other stock index! That is, one takes any two values of the index at any two different dates and calculates how much the index has changed in percentage terms from the earlier to the later. Let us exemplify this by using the second question in your post.
For example, the y value for Sep 1, 2019 seem to be about 33,000, but what does that mean?
Nikkei 225 Total Return Index was 33127.65 as of September 2, 2019 (Sep. 1 was a Sunday). That means someone who bought only one share of each stock in Nikkei 225 as of December 28, 1979, and reinvested any dividends distributed by the stocks in the index back into the index stocks, plus bought and sold the same stocks added to or dropped from Nikkei 225, respectively, at the announced dates in between, would have seen her investment rise ideally by 404.3% as of September 2, 2019 (33128/6569 - 1).
As a second example, consider the index's value as of July 8, 2020, which was 36793.09. Someone who bought one share of each stock in Nikkei 225 as of September 2, 2019, and reinvested dividends distributed as such while buying or selling the relevant stocks as announced again, will have seen her investment rise by 11.1% (36793/33128 - 1) through July 8, 2020.
On another note, if you are interested in Japanese stocks, I recommend following a free-float market capitalization-weighted index similar to S&P 500, such as Topix 500 or Topix (all stocks) instead of/in addition to Nikkei 225. Nikkei 225, while quoted often in the financial media, is quite similar to the Dow Jones Industrial Average, another price-weighted index (also the reason why Nikkei 225's other name is "Nikkei Stock Average") and rather a relic of the past in my view.