To Reach, To Get To, To Arrive At
する verb
たっする
少しおくれたが、無事に目的地に達した。
We were a little bit late, but we arrived safely at our destination.
コウイチのひげは、ついにこしまで達した。
Koichi's beard finally reached his waist.
目標人数に達するまで、あと少しだ。
Only a little longer until we attain the target number of people.
You know that 達 means attain. So, when you add する to it you make it a verb. Now it means to attain.
This reading for this is a bit of an exception. You know the reading for 達 is たつ, but here it's shortened, to just たっ. So make sure you remember.