What is this?

This is the symbol table. Supported symbols are listed here (alphabetically).

Symbols can also be trained here. Just pick a symbol you sometimes need but tend to forget and click it. A canvas will open for your training input.

If there are symbols missing drop me a line or create a pull-request. If you have more specific LaTeX questions though, you will have more success contacting your TeX user group or try the excellent TeX Stackexchange.