ASIC-resistant is the property of a cryptocurrency that is “immune” to ASIC mining.
ASICs are integrated circuits that are created to serve a specific use case, performing a particular computing task. In the world of cryptocurrencies, ASIC devices are designed to participate in the process of mining Bitcoin (or other cryptocurrencies). As such, Bitcoin is an example of a cryptocurrency that cannot be considered ASIC-resistant.
An ASIC-resistant cryptocurrency has its protocol and mining algorithm configured in such a way that using ASIC machines to mine the coin is either impossible or brings no significant benefit when compared to traditional GPU mining. In some cases, using ASICs on ASIC-resistant cryptocurrencies may be even worse than using the more conventional hardware.
Since mining involves multiple attempts of finding a solution for a sort of mathematical problem, the job of an ASIC is to perform as many attempts as possible (i.e., as many hashing functions per second as possible). This means that using ASICs to mine Bitcoin or other Proof of Work cryptocurrencies is much better than using a general-purpose piece of hardware, such as a GPU card.
However, the process of making a cryptocurrency ASIC-resistant is a defensive game, which requires continued development and modifications. This is due to the reason that ASIC designers and manufacturers are constantly producing new models of ASIC miners, and sometimes, the newer models are able to bypass the ASIC-resistance of certain cryptocurrencies.
It is worth noting that blockchain networks that rely on other methods of achieving consensus (such as PoS, dPoS, and PoA) are ASIC-resistant by design. In regards to Proof of Work (PoW) cryptocurrencies, some of them are resistant, and others are not (it depends on the mining algorithm implemented).