About Google Ngram Viewer
When you enter phrases into the Google Books Ngram Viewer, it displays a graph showing how those phrases have occurred in a corpus of books (e.g., "British English", "English Fiction", "French") over the selected years. Let's look at a sample graph:
This shows trends in three ngrams from 1960 to 2015: "nursery school" (a 2-gram or bigram), "kindergarten" (a 1-gram or unigram), and "child care" (another bigram). What the y-axis shows is this: of all the bigrams contained in our sample of books written in English and published in the United States, what percentage of them are "nursery school" or "child care"? Of all the unigrams, what percentage of them are "kindergarten"? Here, you can see that use of the phrase "child care" started to rise in the late 1960s, overtaking "nursery school" around 1970 and then "kindergarten" around 1973. It peaked shortly after 1990 and has been falling steadily since.
(Interestingly, the results are noticeably different when the corpus is switched to British English.)
You can hover over the line plot for an ngram, which highlights it. With a left-click on a line plot, you can focus on a particular ngram, greying out the other ngrams in the chart, if any. On subsequent left clicks on other line plots in the chart, multiple ngrams can be focused on. You can double click on any areaWhat does Ngram Viewer do? of the chart to reinstate all the ngrams in the query.
You can also specify wildcards in queries, search for inflections, perform case insensitive search, look for particular parts of speech, or add, subtract, and divide ngrams. More on those under Advanced Usage.
Source: What does Ngram Viewer do?