Example 1
Hand out these ravioli coupons to whomever you'd like.
Whomever begins an adjective clause that describes the recipients of the ravioli coupons. In our humble opinion, we should be the recipient of all the ravioli. All of it.
Example 2
The concert pianist whose arms were broken in a skiing accident had to cancel his remaining shows.
In this case, whose introduces the adjective clause (whose arms were broken in a skiing accident) that describes the unlucky pianist.
Example 3
Toby watched a documentary that was about marching penguins.
Here, that is a relative pronoun that begins the adjective clause (that was about marching penguins) that likely describes the documentary that was narrated by Morgan Freeman. Morgan Freeman narrates everything, sometimes on helium.