Buffering is a type of shortcut of performing a DM / SDM combo. However, not all characters are able to buffer. To buffer, a character needs a the first part of any of their DMs / SDM to not be one of their moves. From there, buffering is like performing to consecutive moves. Buffering is an important tool if a player is unable to do a DM / SDM motion quick enough in a combo. For a better understanding on buffering, take Kim as an example.

 

Kim's comboable DMs are Hou'ou Hiten Kyaku (qcf,qcf + K) and Hou'ou Kyaku (qcb,db,f + K). At this point, you have to examine to see what motion Kim does not have. Kim does not have a qcf + P / K motion but he has a qcb + K, but not a qcb + P. Now, from here you have to choose which DM you are going to buffer. Take Hou'ou Hiten Kyaku as the example. Kim can use either qcf + P or qcf + K. Immediately after it hits do another qcf + K. This will chain into his DM.

But, things can get a bit complicated...
 

For example, take Joe. Some of his comboable DMs consist of Screw Upper (qcf,qcf + P), Bakuretsu Hurricane Tiger Kakato (qcf,hcb + P). Again, its best to pinpoint what motion that the character does not have. Joe has a qcf + K (Slash Kick) but he does not have a qcf + P. But, he has a hcf + P (Hurricane Upper), so some certain buffers need a bit caution when performing. When performing a close qcf + P, Joe should just attack with a close C. After this has been done, you can either do another qcf + P (for a Screw Upper) or a hcb + P (for a Bakuretsu Hurricane Tiger Kakato).

Normally, characters with both qcf + P / K and qcb + P / K moves tend to have more of a presice motion while the ones with no qcf or qcb motion are simple to get off.