The phrase "to outperform the index" means to achieve better results or higher returns compared to the overall average performance of a particular market or benchmark. Full definition
It turns out that it's very hard for active fund managers and stock brokers to outperform index funds over time, so why pay for them? (tallinvesting.com)
In contrast, an active manager will seek to outperform an index by achieving a higher return, taking lower risk or combining these two techniques. (edwardjones.ca)
Even funds which do not outperform the index in every year, can still outperform that same index over time. (canadiancapitalist.com)