The indexes used in Apache Solr are inverted indexes. In the case of inverted indexing technique, all your text will be parsed and words will be extracted out of it. These words are then stored as index items, with the location of their appearance. For example, consider the following statements:
Mike enjoys playing on a beach
Playing on ground is a good exercise
Mike loves to exercise daily
The index with location information for all these sentences will look like the following (The numbers in brackets denote (sentence no, word no):
Mike (1,1), (3,1)
enjoys (1,2)
playing (1,3), (2,1)
on (1,4), (2,2)
a (1,5), (2,5)
beach (1,6)
ground (2,3)
is (2,4)
good (2,6)
loves (3,2)
to (3,3)
exercise (2,7), (3,4)
daily (3,5)
When you perform delete on your inverted index, it does not delete it, it only marks the document as deleted. It will get cleaned only when the segment, the index is part of are merged. When you create index, you should avoid modifying the index.