In Netrunner, DekCrash is a program that can be used by the runner (the player) to interact with the Corporation’s servers and disrupt their operations. Specifically, DekCrash is a “de-rez” program, which means it can remove ice (security programs) from servers, reduce the Corporation’s credits, and force them to discard cards from their hand.
When the runner installs DekCrash, they can use it to interact with any server. If the Corporation has installed any ice on that server, the runner can use DekCrash to try and de-rez that ice, effectively removing it from play. If the Corporation has no ice installed, the runner can still use DekCrash to force the Corporation to discard a card from their hand or lose some credits.