A hard fork in blockchain technology is essentially an update to the blockchain that invalidates the transactions that happens on nodes that have not upgraded to the latest hardfork. This means that if anyone are running blocks without the newest hardfork update, the blockchain will split to become two blockchains - one running with the latest update and the other without it.
Usually everyone upgrades, and the blockchain without the update just disappears (but the data from it is recorded in the updated blockchain). However, sometimes a certain group wants to keep running the old blockchain without the update, and when this happens we get a split in the blockchain, which is why you can think of it as a fork (it goes from a single line to several smaller ones). This has happened with Bitcoin many times, which is why we have blockchains like Bitcoin Cash, Bitcoin Gold, Bitcoin Diamond, Bitcoin Private etc.
What the hardfork "solves" can range from small bugs that could cause a lot of problems, to huge updates to the entire protocol. I prefer to think of it like an update to any other program, like if you were to update your iPhone to a new version of iOS. You mostly get some new features, but there tend to be lots of under-the-hood changes as well.
As to why Bitcoin has had so many hardforks that have resulted in new/alternative coins is simply because it's the most popular one, and different people have different options as to what is best for the blockchain.
Great explanation. Now I'm clear with this thing also.
Thanks, I'm glad you understand it now :)