This is a letter, written by Albert Einstein, in which he explains the details of a now-famous test of the theory of relativity—an experiment that involves measuring how the Sun's gravity alters the path of starlight traveling towards Earth.
There's a couple of important things to bring up here. First, this sheet of paper is a really handy reminder to test everything—even the work of geniuses. This test had to be performed during a full solar eclipse. Besides that stipulation there was also, you know, the existence of World War I getting in the way. So while Einstein wrote this letter in 1913, nobody carried out and documented the test until 1919.
And that wasn't the end of it. This is the fun thing about science. We might think Einstein is an awfully smart guy. And we might trust the character of Sir Arthur Eddington, who performed the 1919 test. But a hypothesis and a single confirmation of that hypothesis, is not enough to prove even Einstein right. Einstein's theory of relativity rests on several different tests, each carried out by multiple teams, over many years, using increasingly accurate tools. As late as the 1960s and 1970s, this test was still being done with the understanding that Einstein's predictions hadn't yet been verified.
Also very important: Understanding what the hell Einstein's theory of relativity is all about to begin with. For a great in-a-nutshell explanation that puts Einstein into historical context while actually making sense, I recommend checking out the short videos at Einstein Light. Produced by the University of New South Wales, this site combines helpful visuals, clear explanations, and lots of links (so you can take it deeper if you want).