From Forum Nokia Wiki

EPOCROOT is the name of an environment variable that was used on early Symbian OS releases (e.g. EPOC Release 5 and Symbian OS v6.x) for specifying the installation path of the current SDK.

The EPOCROOT represents the part of the installation path between the drive specifier and the epoc32 folder. It starts and ends with the "\" character.

Current Symbian OS releases do not require a EPOCROOT variable to be set in the environment, the "devices" tool should be used instead. However the variable name is still used internally by the build scripts and is often used in discussion boards and articles as an substitute for the actual installation path and/or SDK version that each individual developer may have on his/her development machine.