VRAM or video RAM is a memory that is used by GPU for performing different tasks. Since some algorithms (especially Ethash, which is the algorithm of the most popular mining coin Ethereum) require a larger memory size to save and process the DAG file, some cards with lower VRAM can't mine these algorithms.
For example, ETH DAG size is way over 4 GB, so only the GPUs with VRAM 6 GB or more can mine ETH. On the other hand, ETC has much lower DAG size, so you can mine it with 4 GB GPUs as well.
You can check your GPU VRAM size through the Windows settings.
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