Mid cap is covered in full under Mid-Cap Stocks, which explains the label as it applies to company shares, the usual size range and how mid-sized companies compare with larger and smaller ones. This page explains how crypto markets use the same term.
The "cap" refers to market capitalization, the price of one unit multiplied by the number in circulation. In US stock markets, mid-cap companies are commonly those worth roughly 2 billion to 10 billion dollars, between small-cap stocks and large caps. Crypto data sites apply the same idea to coins, ranking them by market capitalization rather than by company size. In crypto, "mid cap" usually means tokens below the very largest assets but well above the long tail of micro caps, often those ranked somewhere in the top few hundred.
There is no agreed boundary in crypto, so one analyst's mid cap may be another's large or small cap, and rankings shift quickly when prices move. Mid-cap coins usually have reasonable exchange listings and liquidity but are more volatile than the largest assets and more exposed to project-specific risks, such as token unlocks or the failure of a single team.