A broad-market index is an index that tracks the performance of the majority of the companies listed on a stock market, weighted by market capitalisation. This is usually contrasted with a sub-index, which is an index that only tracks companies in a certain sector. Think of a broad-market index as “most of the market” and a sub-index as “some of the market”.

This is discussed in more detail here: ETF: Weighting and single stock risk