What does it mean when you don't know how to pray but you know what you want?
It means a believer has reached a place of infirmity where prayer cannot be articulated, though a desire or idea of what is wanted remains. Scripture says we do not know what to pray for as we ought, so the Holy Spirit intercedes with unutterable groanings. One should be quiet and let the Spirit commune.
Les explains that there come places in life where we just do not know how to pray. We have an idea of what we want, but we cannot put it into words. In that situation, he advises being quiet, even to “shut up,” and let the Holy Spirit commune for us. This is not necessarily a tongues experience; that is not what Paul is talking about. Instead, the Holy Spirit actually intercedes on our behalf with groanings that cannot be uttered. Les also notes this is a different intercessory power than John talks about in his little epistle, 1 John 2.
Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us with groanings which cannot be uttered.