What is the meaning of Bitcoin fork? and why should there be a fork of bitcoin ?
It's when part of the network decides to follow new consensus rules (eg. bigger blocks) that are incompatible with the old ones, leading two a split of the network into two separate ledgers. This leads to owners of pre-fork coins to have a balance on both sides of the split.
Bitcoin got split into BTC and BCH a few weeks ago due to disagreements about the maximum blocksize, ie. how to approach scalability. Other hardforks are caused by other disagreements.
For a more detailed explanation see for example here:
https://www.coindesk.com/short-guide-bitcoin-forks-explained/Isn't this the kind of things a team should agree upon BEFORE launching a big project?
(I'm new to this, it's a real question)
Well, that's the thing though. For most of Bitcoin's lifetime there was agreement on how to proceed with the project. However Bitcoin is not a centrally directed entity such as a regular cooperation. It follows that there's no thing such as a final word by the CEO of Bitcoin.
So what happens when there's a dispute in which direction the project should go? You start your own and let it compete in the open market. So that's what happened with Bitcoin. A group of developers decided that they didn't like the direction Bitcoin was going -- namely the way scaling was handled -- so they decided to build their own version of Bitcoin. Some miners and parts of the community felt the same, so they supported the new hardfork -- in this case BCH -- by buying up coins and mining the BCH blockchain.
And that's where we are right now.
The story is more or less the same with the other hardforks, only that the reason of dispute varies and the amount of miner / community support.
It's also more or less the story behind alts -- Some dev likes Bitcoin but sees room for improvement that wouldn't fly with the rest of the Bitcoin devs, community and miners, so they start a coin of their own. Sometimes from scratch, sometimes on the basis of Bitcoin, sometimes on the basis of some other alt before them.