What does (block cipher) decryption parallelizable mean? I've tried searching this answer on here and on google, but I cant seem to find a succinct answer (maybe because its obvious). 


My understanding from wiki/tutorialspoint/ppt is that for CBC/CFB, decryption parallelizable means a plaintext block can be recovered from two adjacent ciphertext block. Is this correct or is there more? I really don't see how the word parallel fits into this at all. 
It is stated as advantage for the decryption to be parallelizable, but if an attacker has access to your ciphertext, then couldn't the attacker just decrypt it easily by using adjacent ciphertext blocks.
I've know about C threading, is this related to it? As in you can decrypt the from multiple threads. I don't see how this is possible either because it seems to be done from one chain.
plaintext block can be recovered from two adjacent ciphertext block


You also need the key. For example, for CBC mode, to decrypt block $i$, you need ciphertext block $i$, ciphertext block $i-1$, and the key. You do not have to have made any calls to the cipher on any previous or future blocks in order to decrypt block $i$.

Contrast this with OFB which is not parallelizable. In order to decrypt block $i$, you need to have run the cipher on block $i-1$ first.


  if an attacker has access to your ciphertext, then couldn't the attacker just decrypt it easily by using adjacent ciphertext blocks


No, because the attacker does not have the key.


  I've know about C threading, is this related to it?


Threading is one way to parallelize. In theory, with CBC or any other parallelizable mode, you could decrypt different blocks in different threads, processes, or even different machines.