A tombstone is a marker Cassandra writes when you delete data. Deletes can't remove data from every replica at once, so Cassandra records the delete as a new write with a timestamp.
A tombstone is kept for the table's gc_grace_seconds, which defaults to 10 days. That grace period lets a replica that missed the delete receive it through repair, so the deleted data can't come back as a zombie. After the grace period, compaction removes the tombstone and the data it covers.
Too many tombstones slow down reads, because a read has to scan past them.
Rewriting in plainer words…
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