what does "blocks" mean in the context of HDFS ecosystem? is it the same concept in the context of a single hard disk drive? According to Hortonworks


  HDFS has demonstrated production scalability of up to 200 PB of
  storage and a single cluster of 4500 servers, supporting close to a
  billion files and blocks.


what does "blocks" mean in this post?
Essentially yes but the size is very different. A hard drives default block size is 512 bytes but Hadoops default block size is 128 Megabytes.

Hadoop also replicates blocks to maintain integrity of the data.