What does “⑂ block became a side fork” mean in Geth?

Question

Two Geth nodes were mining in the same private net, and one of them gave ⑂ block became a side fork. I don't get what it means.

I'm new in Ethereum. I'd appreciate it if you give me an explanation for newbies.

Answer 1

Both mining nodes are competing to create blocks, and mined a block at around the same time. Both blocks are valid, but they are 2 possible realities. They keep mining and one of them finds another block. The longest chain (2 blocks instead of one) becomes the accepted reality, and the shortest chain (1 block only) is a side fork, it is useless.

It is normal that different nodes can come up simultaneously with solutions, especially as the block generation time is low (it is more likely in Ethereum than Bitcoin for example).