I can write out something lengthy a little later but short of it is this...
1) Index Funds are funds that are allocated in a way to mimic a particular market/index. For example, if you buy the Vanguard S&P 500 Index Fund then the fund is set up to purchase share of companies and have the holdings reflect the actual makeup of the S&P 500 exchange. The upside is that this is much easier to manage so fees are usually much less compared to your typical mutual fund. The managers aren't picking winners and losers in the stock market.
2) You can usually invest in these the same way you currently do in your 401k. Go to your Investments selections where you probably have some sort of targeted fund (2055, 2060, etc), you can choose index funds instead. Like Small Cap, Emerging Markets, S&P 500, etc.