explanation about these three line of code

What specific device part number are you using?

What software release did you pull these excerpt from?  Please cite your source.

Princess John said:

 BTCTL = 7;                                                                           // Configure BT interrupt rate

This is a module configuration. I don't know BTCTL (which device are you using?), which likely is the/a control register of the BT module (sounds not familiar) but it sets it to a value. And according to the comment it is a setting for the interrupt rate of this module.

Princess John said:

IE2 |= BTIE;                                     // Enable BT interrupt

This line enables the interrupt for the BT module, by settign the associated bit in the global Interrupt Enable register #2. If this bit is not set, no interrupts will occur when the BT module issues an interrupt request.

Princess John said:

  _BIS_SR(GIE);// global enable interrupt

This sets the GIE (Global Interrupt Enable) bit in the status register. If this bit is not set (which it isn't after a reset), no interrupts are accepted at all, independent of their source and individual configuration.

one more questions,

if somewhere I can find the definition of  "_BIS_SR()" function.

For example, I have some sample code like this : "_BIS_SR(LPM0_bits + GIE)";

I curious to know how " _BIS_SR()" function woks with LPM0 module and GIE

It is important to know what toolchain you are using.  The documentation for that toochain will tell you how this macro is defined.

For Code Composer Studio, _BIS_SR() is defined in a header file called in430.h which is found in the CCS installation directory in <CCS_INSTALL_DIR>\ccsv4\msp430\include

This macro is defined as :

#define _BIS_SR(x)                      _bis_SR_register(x)

The C Compiler User's Guide for CCS, document number SLAU132, indicates _bis_SR_register(unsigned short mask) is an instrinsic which maps to the assembly instruction: BIS mask, SR

This assembly instruction is described in the MSP430 Family User's Guide in the Instruction Set section, as performing a bit set of a register, which in this case are the bits defined in "mask" on the Status Register, SR.

BrandonAzbell said:

_bis_SR_register(unsigned short mask) is an instrinsic which maps to the assembly instruction: BIS mask, SR

Small correction:

BIS #mask, SR

:)