The danger
A split brain happens when a network partition cuts a cluster in two and each side believes the other is dead. Both elect a leader, both accept writes, and when the partition heals the two histories conflict. This can corrupt data badly.
Quorum is the cure
The standard defense is a majority quorum. A side may act as leader only if it can reach more than half the nodes. Since both sides cannot each hold a majority, at most one side stays active and the minority side steps down.
Why even node counts hurt
A cluster of four can split two and two with no majority, so nobody leads. Odd numbers like three or five avoid this. A lightweight witness or tiebreaker node can give an even cluster a deciding vote.
Belt and suspenders
Quorum prevents two leaders accepting writes, but a slow old leader might still try. Fencing tokens stop its late writes at the storage layer, and STONITH style fencing physically powers off the stale node.
Key idea
Split brain is prevented by requiring a majority quorum to lead so a partition can leave at most one active leader, backed by fencing against stragglers.