There are two types of fat associated with cuts of beef (or lamb) – intramuscular fat and external fat. Intramuscular fat, or marbling, gives flavour and keeps the meat moist.
External fat is found around the outside – if you like your beef to be as lean as possible, make sure that you trim any excess external fat before cooking.
All you need to trim your beef is a sharp knife – simply slice away at the fat gradually (without slicing into the meat) until there is only a very thin, even strip of it remaining.
Leaving this thin strip ensures that your meat will be moist and juicy.
For more great information about how to trim the fat off steaks, see The Spruce.