The S&P 500 is an index of the 500 largest U.S. companies. It is market-capitalization-weighted based on those companies' values. When people talk about "the market," they are often referring to the S&P 500 (other times the "Dow" which is an index of just 30 stocks).
Michael Batnick, Director of Research at Ritholtz Wealth Management, took the finance world by storm recently by putting together the following chart and data. He points out that the market cap of the top 5 S&P 500 companies is $4,095,058,706,432 while the market cap of the bottom 282 S&P 500 companies is $4,092,769,755,136 (as of July 2018).
What we can learn from this is that there are a few select companies that can control the narrative of "the market." These are companies that we all know, recognize, and may even love. But that should not dictate how we invest.
It's true that FAANG and other mega-cap stocks have performed well, but so have companies in other deciles of the S&P. It is just as important to remember that there are about 3,000 other publicly-traded companies in the US and thousands more abroad.