Large cap is covered in full under Large-Cap Stocks, which explains how the label is used for shares, the usual size thresholds and how large companies behave compared with smaller ones. This page explains how the same label is used in crypto.
"Cap" is short for market capitalization, the total value of all units in circulation at the current price. In the stock market, large-cap companies are commonly those worth more than about 10 billion dollars. Crypto data sites borrow the idea but rank coins rather than companies, multiplying each coin's price by its circulating supply and sorting the results. "Large cap" in crypto usually refers to the coins at the top of that ranking, such as Bitcoin and Ethereum, and sometimes to the top ten or twenty assets.
There is no official cut-off in crypto, and the boundary moves with the market. Large-cap coins tend to have deeper liquidity and more exchange listings than mid cap or micro cap tokens, but they can still be highly volatile. Because market capitalization uses the last traded price, it can overstate what holders could actually sell for, especially for tokens with a small free float.