Hi finally i got .
TMOD means .It decides the source of clock for the timer. it has certain register which decide the working of timer see Ali Mazdi's text page number 255.
to take Timer we need to clear it first that is TR0 (one of the timer in 8051) ie TR0 =0000h;
in .ASM it is ,SETB TR0 .(0000h)
The timer register value counts up or increase in accordance with its machine cycle that is 0001,0010,0011 (binary)etc
atlast it will reach FFFF again it go to 0000h while the flag register will set ,so we can count TF register as a counter or else .