The phrase that which (or those which, in plural) and the word what can be used as pronouns to introduce a relative clause. The two alternatives are nearly equivalent in meaning, although that which is more formal than what.

Our conclusion is very much in line with that which others have made.

Our results are very much in line with those which have been found previously.

Our results are very much in line with what others have found.

In some cases, that which is clearly preferable to what.

When he first meets Daisy, his image of her and that of her house are closely connected because to him they both represent that which is forbidden.