Chain link plier
It is common safety practice not to join the chain by re using the pins. This is especially true for higher speed chains where the out plates are thin. The solution is to use the "quick links". But to install and remove these "quick links", you will need this tool. These pliers are compatbile with the "quick links" by various chain makers, inlcuding Shimano, KMC, SRAM, YBN, Sun Race, just to name a few.
Note that the "quick links" typically have a limited number of times that they can be installed and removed. If they become loose enough such that they can be installed and removed by bare hand, it is likely that those "quick links" have worn and it's time to replace.