My auto transformer data sheet received from manufacturer has some parameter which i dunt understand.

It is

Short circuit impedance referred to the first winidng

At principal tap

High voltage -to low voltage 29.75 ohms per phase
Low voltage -to Tertiary 23.65 ohms per phase
High voltage -to Tertiary 99.6 ohms per phase

what does this mean?and whre can i use this parameter??

As I can see, you are talking about regulating 3 windings autotransformer and its equivalent short circuit impedance which are used in equivalent schemes of autotransformers in power systems...
Principal tap represents middle tap of regulating switch under rated voltage.

Explanation of "High voltage -to low voltage 29.75 ohms per phase":

Z12=(uk12[%]/100)*(Un1*Un1)/min{Sn1; Sn2}=29,75 Ω

Explanation of "Low voltage -to Tertiary 23.65 ohms per phase":

Z23=(uk23[%]/100)*(Un1*Un1)/min{Sn2; Sn3}=23,65 Ω

Explanation of "High voltage -to Tertiary 99.6 ohms per phase":

Z13=(uk13[%]/100)*(Un1*Un1)/min{Sn1; Sn3}=99,6 Ω

where are:

Z12, Z13, Z23-short circuit impedances of regulating 3 windings autotransformer,
uk12[%], uk13[%], uk23[%]-short circuit voltages of regulating 3 windings autotransformer expressed in percents,
Un1, Un2, Un3-rated voltages of regulating 3 windings autotransformer,
Sn1, Sn2, Sn3-rated (apparent) powers of regulating 3 windings autotransformer.

After that, you can calculate short circuit impedances for each winding of regulating 3 windings autotransformer by using next expressions:

Z1=0,5*(Z12+Z13-Z23)
Z2=0,5*(Z12+Z23-Z13)
Z3=0,5*(Z13+Z23-Z12)