The Collision Differential for MD4 - a question on notation (Wang, et al)
In the MD4 algorithm, the message which is being hashed is split into a series of 512-bit blocks. The collision attack which you reference forms a collision in a single block. That is, the attack forms two colliding messages, X and X', which are identical for every block but one. The paper refers to the blocks of X and X' which differ from one another as M and M', respectively. 

Because the parts of X and X' which come before blocks M and M' are identical, they have the same hash. The hash of these blocks is referred to as H$_0$. So $\Delta$H$_0$ = 0 is just saying that the intermediate hash of the messages before blocks M and M' are identical.

$\xrightarrow{(M_,M')}$ means that block M is hashed as a part of its message, and block M' is hashed as a part of its message.

Once these message blocks have been hashed, the hashes of the two messages are still identical, which is what $\Delta$H = 0 is saying.

So, what the equation as a whole is saying, in English, is: "The intermediate hashes of two messages are identical. Then, we process a single block of each message. These blocks are called M and M'. Even though these blocks are not identical, their final hashes are identical."

Note that in some papers for differential cryptanalysis, M and M' will refer not to a single message block, but to the message as a whole. So this is not standard notation. 

When I was starting out in differential cryptanalysis, I also had a hard time understanding this equation, but I hope this has helped you.