CRISPR is a tool that allows you to cut the DNA in two disjoint pieces at a specific point (specification of this point is a parameter of a particular CRISPR instance). What happens then depends on your setup; bacteria will just insert some junk at that break point, or you can pack your custom DNA sequences along the CRISPRs and they will be spliced in, connecting to each of the two disjoint pieces by one end. Thanks to this, at that specific point, you can disable a gene or modify or add an extra sequence.
We had tools to do this before - restriction enzymes or TALENs. They weren't really usable for therapeutic purposes, though, due to much less reliable targetting, more laborous engineering (parametrizing your instance for a specific sequence) and low effectivity (the break happens only in a a few percents of cases). CRISPRs are easily parametrized, can be precisely taretted, and have effectivity in tens of percents (in general; can vary organism by organism). It's still a work in progress, but looks pretty promising!