This event is seen generally when multiple session are trying to read the same block or multiple session waiting for a change to complete in the same block.
Please read this MOS note -> WAITEVENT: "buffer busy waits" Reference Note (Doc ID 34405.1)
That note above will answer your first two questions.
The following MOS notes will answer your third question and make you understand this wait event event better.
Resolving Intense and "Random" Buffer Busy Wait Performance Problems (Doc ID 155971.1)
Understanding and Tuning Buffer Cache and DBWR (Doc ID 62172.1)
Resolving Issues Where Waits for 'buffer busy waits' are Seen When Concurrency for Buffers is High (Doc ID 1476043.1)
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