What does the plantar surface of the distal forelimb mean in simple terms when reffering to bpm a dog?

"The plantar surface of the distal forelimb" is the simple terminology, although it is medical terminology rather than common-use English.

The plantar surface is the side of the foot where the pads are; the distal forelimb is the part of the front leg that is furthest from the shoulder.

Based upon the reference to "bpm", I am guessing you are looking for directions on palpating the pulse of the dog. You should gently slide the tips of your fingers over the hollow spot at the joint between the paw and the last long leg bone. There are two small but superficial (close to the skin) arteries there that you can measure a pulse rate from.