Splicing is using Plasmids to alter your genes. It takes its name from 'splicing', or combining, new DNA to your own DNA. That enables you to use Combat Plasmids, such as Electro Bolt, or Gene Tonics, which are passive Plasmids that protect you.
Splicing too much results in various physical mutations, mental degradation, and an addictive dependence on ADAM, the stuff Plasmids are made of. If a person splices too much, they become a 'Splicer'.