REIT stands for Real Estate Investment Trust. It's basically just a company that owns real estate. Some specialize in certain kinds, like commercial, residential, storage units, etc. There's some different rules for companies that are organized as REIT's. They have to pay more of their earnings out as dividends. So REIT's tend to have a high dividend yield.
The VTSAX you own already has a small % of REIT's in it, so you already have some. Some people like to have more REIT's in their portfolio, and so might buy a REIT specific index fund, such as VGSLX. REIT dividend are mostly not "qualified dividends" for tax purposes.
When your portfolio is larger you may want to do that, or overweight certain other assets. Before you do that though, i would recommend continuing to save, and then learn more about asset allocation and asset diversification. You have a good portfolio for a beginning investor. But its a little small for further diversification. At this point most of your gains are likely to be from saving, not better returns from a better portfolio.