This is an R Markdown document. Markdown is a simple formatting syntax for authoring web pages (click the MD toolbar button for help on Markdown).
When you click the Knit HTML button a web page will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this:
x = 1 + 3
x = c(1, 2, 3, 4, 5)
y = c(2, 4, 6, 8, 10)
plot(x, y)