We use wat:

• to refer to a whole phrase: "It is raining, which makes her sad"
• without an antecedent (or to refer to an invisible dat?): "[That] what goes around, comes around"
• to refer to alles? (everything, all), iets & niets? (something and nothing), or weinig (little, few).
• to refer to an independent indefinite pronouns? or adjective, e.g. het enige, het mooie (the beautiful thing), het grootste (the biggest thing).

Wat referring to a whole phrase

When we refer to a whole phrase, we always use wat:

Het gaat deze zomer veel regenen, wat slecht nieuws is voor de toeristische sector.	It is going to rain a lot this summer, which is bad news for the tourist industry.
Ze letten goed op hun gezondheid, wat niet betekent dat ze nooit eens patat eten.	They look after their health, which does not mean that they never have fries.

Wat without antecedent

Just like in English, we can use wat without an antecedent. You could say that it refers to an invisible dat?.

[dat] Wat ze zei, was zeer schokkend.	[that] What she said was very shocking.
[dat] Wat we vergaten, was dat er een uur tijdsverschil was.	[that] What we didn't remember was that there was an hour time difference.

Wat referring to alles, iets, niets, or weinig

Dat is iets wat ik me nog goed kan herinneren.	That is something that I recall very well.
Is dat alles wat je hebt gedaan?	Is that all that you have done?
Ik heb niets gevonden wat het probleem kan verklaren.	I have not found anything that could explain the problem.
Er is maar weinig wat ik niet lust!	There are only very few things that I do not like (to eat).

Wat referring to independent indefinite pronouns and adjectives

In English, adjectives are normally linked to a noun or pronoun. That is why we add 'thing' to the English translation.

It is also acceptable to use 'dat', just like in English. However, there is a strong preference for 'wat'.

Het laatste wat ik me kan herinneren, is dat ik tegen die auto aanbotste.	The last thing that I can recall is that I collided with that car.
Het eerste wat ze na de wedstrijd zei was dat ze honger had.	The first thing that she said after the match was that she was hungry.
Dat is het het enige wat ze zei.	That is the only thing that she said.
Dat was wel het vreemdste wat we ooit hadden meegemaakt.	That really was the weirdest thing we ever experienced.
Dat is het minste wat je van ze mag verwachten.	That is the least that you could expect from them.

Wat turning into waar

If wat is preceded by a preposition, it turns into a pronominal adverb?: We substitute wat for waar and let the preposition follow after it.

preposition + wat

waar+preposition

Het gaat deze zomer veel regenen, waar we niet blij mee zijn.	It is going to rain a lot this summer, which we are not happy with.
Ze letten goed op hun gezondheid, waar ze op de lange termijn voor zullen worden beloond.	They look after their health, which they will be rewarded for on the long run.

Wat without an antecedent (see above) never turns into a pronominal adverb.