Its says use rounding or compatible numbers to estimate the sum and example 198 + 727 ans is 200+725=925 i still don't understand how to do it. heres a problem 87+34 and here is one 222+203

Okay, rounding is when you take a number such as 198 and imagine that it is something easier to do math with, like 200. 
200 is easier to add with because there are zeroes in the tens place and the ones place. 
725 is easier to add with than 727 because the ones place digit, (5), is an easier base to use, just like zero. 
Using the easier numbers, 200 + 725 = 925
Using the original numbers, 198 + 727 + 925
*Because you took two pieces from the 727 and added them to 198, the four numbers in this problem are compatible. 
Compatible numbers are when you use numbers that are close to the original ones and make it easier to do the problem in your head. 

In the case of 87+ 34, the simplest numbers to use would be 90 instead of 87 and 35 instead of 34. Not as easy as the first equation, but still doable. 
90+35+125
    or
87+34=121

In the case of 222+203, the simplest substitutions are 220 and 200. 
220+200=420
or
222+203=425
Hope this helps, 
Ahawk