ASIC is an acronym for Application Specific Integrated Circuits. These are specialised electronic chips or ICs (integrated circuits) which are meant to do a specific operation in the most efficient manner possible. They are designed to perform a single desired operation at a very high speed consuming minimum power. General purpose CPUs or GPUs on the other hand are far more versatile and are designed to execute complex instruction sets.
Cryptocurrency mining usually involves solving cryptographic puzzles. These puzzles are computationally intensive yet are repetitive. ASICs being architecturally and functionally simpler than CPUs are much better suited if we need to perform a specific operation over and over so are best suited to the task of cryptocurrency mining.
Since, ASIC Hardware gives advantage to a specific set of people who can procure or build ASICs at much more ease than most others, few cryptocurrencies like Ethereum use ASIC resistant hashing algorithms (like Dagger-Hashimoto) making it fair to everyone.
ASIC or Application Specific Integrated Circuit is a microchip exclusively to perform one task at a time. ASIC mining is a crafty method of mining various coins at a much faster rate than any normal desktop or laptop.