What is the function of
approveFrom method in the ERC20Token interface and how do the functions
allowance relate to
approveFrom does not exist. You might be referring to transferFrom.
transferFrom() allows a 3rd party, which could be another person or even another contract to transfer tokens from someone's balance to someone else.
Of course, given this is a public function, if anyone could transferFrom anyone's account to another account, any amount of tokens, the system would be broken.
That's where approve() comes in. Approve allows the owner of the balance to set a maximum number of tokens the approved party may use on his behalf. This would allow, for example, for me to approve a contract to spend x tokens on my behalf.
allowance() is just a function to read how many tokens x account has approved y account to use on his behalf.
You will find a answer in ERC: Token standard. Hope it helps~