The AUTOEXEC.BAT file is a reference to original MS-DOS systems which would load settings from a file called CONFIG.SYS (which had to be located in the C:\ path), and then execute any commands found in the AUTOEXEC.BAT (also in the C:\ path). Commonly the AUTOEXEC.BAT file would define the system PATH, load any drivers, and launch Windows. Because DOSBox already emulates most of what a CONFIG.SYS, and AUTOEXEC.BAT would do, there isn't much need to define anything other than your mountable drives.
The AUTOEXEC.BAT is a segment of the dosbox.conf file in the program directory of your DOSBox installation.
A Common AUTOEXEC.BAT
@ECHO OFF MOUNT C C:\DOSGAMES C: CLS ECHO Starting MS-DOS 6.22...
Starting MS-DOS 6.22...
(The message that MS-DOS printed immediately before loading the CONFIG.SYS and AUTOEXEC.BAT).
MOUNT C C:\oldgames C: