It means that you have two versions of CRS running at the same time. This can happen if the devices on your network come up slower than your services. If CRS detects it, they all shut down and the database does not come up. Put in a sleep init.crs file in your rc.d directory (if UNIX) to delay RAC startup until all your devices come on line.
Split brain means: -
consider this simple scenario: 2 nodes, the interconnect is implemented with one dedicated switch and we have one voting disk. Now if for example the switch fails, the interconnect would not be able to work any more. no more cache fusion, no more exchange about cluster configuration and (more importantly) changed blocks/data. But both instances are still fine and could access the database. That would be a split brain – worst case would be both instances update the same blocks in the database which would lead to data corruption.