Collision Resistance is a property of cryptographic hashing functions that help avoid the possibilities of identical hashing outputs from different inputs.
A proper hashing algorithm is designed to be one-way, meaning that every input will have its own unique output; a hash that identifies the input data. If two different inputs has the same output, then the algorithm is compromised because if two entirely separate things represent the same identity then whenever that identity is brought up, there will be an error in unwinding it. Key note; collision resistance does not mean that collisions are certainly not going to happen, there is always the microscopic random chance of collisions happending.
In blockchain and cryptocurrency, collision resistance is of the utmost importance. If two key pairs signify the same wallet, one of the keys can steal the assets that are in the wallet and transfer them elsewhere.