Accessing to (your) bitcoin requires a private key.
This is kind a long number password to access the bitcoin on the network called blockchain.
All bitcoin are saved in this (online) network.
A private key looks like this:
5Kb8kLf9zgWQnogidDA76MzPL6TsZZY36hWXMssSzNydYXYB9KF
Important thing is you save your private key on different places in case you need it back again. One good practice is print it on a paper with QR code. This is called Cold Storage. This is your backup.
Don't show this private key to anyone else and save a copy somewhere else than your home.
Act like a bank, because with bitcoin you are own bank.
When your PC is gone, you take the private key and setup a new wallet on a device. Amount of the bitcoin will be exactly the same as it was left last time, because it is saved on the blockchain.
Read more here
https://en.bitcoin.it/wiki/Private_key
2Note that if your device gets stolen and the thief gets access to the private key he can transfer the bitcoins and after that, your backup is not going to help you. – Jannes – 2015-08-15T17:09:28.217
1You can encrypt the wallet with a passphrase to protect against that, however then you risk losing the bitcoin if you forget the passphrase. – ryanc – 2015-08-21T03:53:00.743