Every column value can be either a dot or the same column character. Those columns are boolean and the column character stands for "True" while the dot stands for "False".
- R stands for "Returns" and is True (has an "R" in the column's value instead of a dot) if the function returns. Functions may not return and this requires special handling by IDA and the reverse engineer.
- F stands for "Far" and is true for functions that are far functions, or false otherwise.
- L stands for "Library" function, and is true for functions IDA recognised as library functions (usually using FLIRT).
- S stands for "Static" function.
- B stands for a E/R/BP stack frame based function. As part of IDA's analysis functions that use the xBP register for easier stack access are marked so.
- T stands for Type and indicates whether a function has type information assigned. Either manually or automatically by IDA using FLIRT.
- the equal sign (=) indicates the frame pointer is equal to the initial stack pointer.
All of those values can be manually accessible using the "Edit function" dialog (alt+p), like seen here:
This is also documented and can be found at Hex-Rays website, with a shorter explanation.
Additionally, according to Arnaud Diederen:
since 7.1, IDA also provides tooltips for those arguably cryptic single-letter columns. Just hover your mouse over the column header for a couple seconds, and the tooltip should appear.