ASICs, or application-specific integrated circuits, are silicon chips designed specifically for a particular use. In the case of cryptocurrencies, that use is mining.
ASIC miners are purpose-built machines that do nothing but mine for a specific cryptocurrency.
ASICs for Ethereum do exist, but they’re not particularly common or widely available. That’s because Ethereum mining is still reasonably profitable with just a regular old GPU.
ASICs are only worth it if you’re looking to mine on a large scale with industrial-level hardware.
So, if you’re just a hobbyist miner looking to eke out a little more ETH, an ASIC miner is probably not for you. But if you’re looking to set up a large-scale mining operation, then an ASIC rig might be worth considering.