DNA base editors are engineered ribonucleoprotein complexes that can effect single base substitutions in DNA. They are composed of a base-modification enzyme (usually a deaminase) plus a catalytically impaired Cas9 nickase which cannot make double-strand breaks. Binding of the guide RNA to target DNA results in the creation of a single-stranded DNA region that can be acted upon by the base-modification enzyme. The Cas9 nickase nicks the unedited strand which stimulates biased repair in the correct direction.
figure from https://blog.addgene.org/single-base-editing-with-crispr
BE3 is a cytosine base editor composed of rat cytidine deaminase APOBEC1 (which changes C to U), Cas9 nickase, and uracil DNA glycosylase inhibitor which helps prevent base excision repair from U back to C.