what is forking attack? and different between forking attack and selfish mining? I am a student who studies block-chain for these days.

I have a question about forking attack and selfish mining in mining strategies 

as you see picture below



I don't understand why miner should send some money to a victim to do forking attack...

what does this means to do forking attack? 

is it possible to do forking attack except sending some money to a victim?

and as below picture about selfish mining 



what is difference between forking attack and selfish mining if miners exclude 

to do send some money to a victim in forking attack?

can anyone help me to understand what M->B and M->M'(double spending) means??

and why do need it to do forking attack??
With 51% of the network hashrate the forking attack is theoretically possible. Classically, what you have described is a double spending attack.

First, the attacker pays M->B and B releases the goods. Then, the attacker presents a longer valid chain paying M->M instead and the blocks containing M->B are orphaned.

It is not simply a matter of presenting an alternate block, it must be a valid alternate block(s) with more valid proof of work (a longer chain) which must also be based on the genesis block and any (each) checkpoint.

With 51% of the network hashpower, selfish mining is also possible, where the attacker ignores any network generated blocks and continues to extend their own chain with blocks that they have created. Because the attacker's hashpower is greater, theirs will be the longest chain with the greatest PoW.