The answer by @user1668782 is a great explanation for the question and I'll try to give a graphical depiction of it.
Assume we have a file of 400MB with consists of 4 records(e.g : csv file of 400MB and it has 4 rows, 100MB each)
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If the HDFS Block Size is configured as 128MB, then the 4 records will not be distributed among the blocks evenly. It will look like this.
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Block 1 contains the entire first record and a 28MB chunk of the second record.
If a mapper is to be run on Block 1, the mapper cannot process since it won't have the entire second record.
This is the exact problem that input splits solve. Input splits respects logical record boundaries.
Lets Assume the input split size is 200MB
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Therefore the input split 1 should have both the record 1 and record 2. And input split 2 will not start with the record 2 since record 2 has been assigned to input split 1. Input split 2 will start with record 3.
This is why an input split is only a logical chunk of data. It points to start and end locations with in blocks.
Hope this helps.