What does it mean that for bitcoin block to be on the Main Chain? What other chains are there? Is this as opposed to testnet?
This is regarding the main_chain value in the blockchain.info api here: https://blockchain.info/api/blockchain_api
If a block is part of the main chain, that means that the tip of the blockchain is a descendant of that block.
If a block is not part of the main chain, that means there is a longer chain of blocks that does not include this one. These are called orphaned blocks or invalid chains.
If a block is in the main chain, its parent is automatically in the main chain too.
In the above figure, the black boxes are the main chain. The grey blocks are invalid chains. The green box is the genesis block.