This setup below is part of the b6 system chess opening for Black and is occuring quite often so you should know how to handle it.
In this B6 System Setup White plays his bishop to d3 in his third move already to protect the e-pawn with his bishop instead of playing the knight to c3, which is the usual move.
As the white bishop moved out very early it can be attacked by playing knight c6 followed by Nb4 (see second diagram) to trade off this dangerous bishop that is aiming at the black kingside and can be used for future attacks.
As you can see the bishop can not move somewhere else or you capture the e-pawn with your bishop at b7. After you have traded off this fast moving white bishop Black has the two bishops.
In the following games you will see that White players often try to stop Black from playing Nb4 by playing a3 or c3 to preserve the bishop. The move a3 can be considered a waste-of-time move so Black just continues to develop his pieces, for example knight to f6.
The move c3 is passive as well, so Black follows with the natural center move e5 and tries to put his undeveloped bishop f8 to c5, where it is placed very effective. But this is not always possible, of course, but just take this as a guideline. You have to play d6 eventually, so it is important to get your bishop out and place it before your pawn chain e5 d6, as your center pawns are sitting on black squares and reduce your bishops power if the bishop is placed on e7, for example.
Just get this: Your bishop f8 will be a dead bishop because of your pawn structure, so get it out to c5 or b4 and get rid of it. Trade it off for the white bishop that might go to e3 or force White to trade it on c5.
The position below is likely to happen in your Bishop d3 setup games, so have a look.
In the position above Black captures the e-pawn with his knight 1...Nxe4!! 2.BxB NxQ 3.BxQ NxN+ followed by recapturing the white bishop at d8
Replay the games below and observe and understand what will happen with the black bishop f8 and how the bishop on b7 will be brought to life.