Crypto currencies inherit their security from a process called “Proof of Work”. In short, it’s all about trying to find a special number, a blockhash, with a special value. This requires lots of work by specially designed machines. It is this work that strengthens the network, and secures the value of the crypto currency.
Another form of securing the network is the more eco-friendly alternative “Proof of Stake”, where the holder uses crypto currency collateral to achieve security for the blockchain instead.