how to understand this sentence.

Intelligence gathering efforts focused on such groups (supremacist and radical groups) have largely failed

Efforts by (presumably) government agencies to gather intelligence (information) about supremacist and radical groups have mostly failed.

and military-style organization and training are characteristic of a number of the groups that are known

A number of the groups that the government agencies know about are organized and trained using similar methods to those used by official military forces.

especially those with a survivalist bent.

The word "bent" (noun) means "tendency". A person of a practical bent is a person who is good at practical things. A person of a nautical bent is interested in, knowledgeable about, or experienced in boats and ships and the sea. Survivalists are (mainly American) people who believe that some kind of disaster is likely to happen (e.g. nuclear war, epidemic, civil disorder, judgement of God) and that in order to survive they will need to collect guns, ammunition, food, etc, organize themselves like an army, prepare bunkers, shelters, caves to hide in while the disaster takes place. So a group of people of a survivalist bent is a group of people who share that belief.