an example:
contains keywords the cat is on the table
will match these value in the database (just some example):
cat table
CAT table
the cat is on the yellow table
the table is a cat
your cat is not a TABLE
I think my TaBLe is a Cat
one cat plus one cat equals a table
many cats on the table
tables cat
tables cats
but it will not match:
the cat is on the tablet
catss table
i just have one cat
the table is empty
I hope this helps 
Cheers
Mariano