Level 1 data typically will display the Best-Bid-Offer ("BBO" or "Inside Quote"), i.e. the lowest ask and highest bid available at the time.
Level 2 data displays the best bid and ask prices (also known as "top-of-book") for each market participant in a given security.
Level 2 data will display the highest bid and lowest ask for each individual market maker.
Level 2 information is of interest to traders and brokers because it indicates the buying and selling pressure behind individual securities.
Because of the efficiency of the SETS system, stocks traded tend to have narrow spreads so the cost of entry and exit from a position is much smaller.