You are in fact indexing the DataGridView by using that syntax.
In your case it says: Show the string of the Value in column a and row b. (column and row indexes are zero based, so column 0 would be the first column)
Glad to be of help.
I will give you one more tip: don't use the names a and b use something like c or ror better still coland row. Your code will become clearer and easier to follow.
Happy programming :)