It means that when both sides of an equation have the same number subtracted from them the remaining expressions are equal. ( ......Hope that I helped you...... )
The subtraction property of equality means that a=b and c=d then a-c=b-d or a=b and c=c, then a-c=b-c basically when 2 things are equal, and you subtract the same value from both sides, the equation remains equal
or a+c=b=c simlifies to a=b where you can simlify the same thing from both sides and retain the equality