I touched on this topic when teaching in Romans 8:

Romans 8:26-27 In the same way the Spirit also joins to help in our weakness, because we do not know what to pray for as we should, but the Spirit Himself intercedes for us with unspoken groanings. And He who searches the hearts knows the Spirit’s mind-set, because He intercedes for the saints according to the will of God. Note how this couplet starts out – in the same way the Spirit also joins to help. In what same way does the Spirit help us? In verse 26 we read that He helps us when we are too weak and ignorant to know how to pray. This opening phrase must refer to what was just previously written, so we can rightly conclude that the Spirit of God helps to wait eagerly with patience. If we think we can do those things, we think too highly of ourselves and forget the flat teaching of Jesus – that apart from Him we can do NOTHING. There is no wiggle room in that statement. This goes hand-in-hand with the familiar verse in Philippians 2:13 – for it is God who works in you, both to will and to work for his good pleasure.
As the Holy Spirit helps us in our weakness to eagerly look for the return of Christ while waiting patiently for Him, He helps us pray – because we do not know what to pray for as we should. There are various translations of this phrase and the most agreement is that we do not what we should pray for – we lack the wisdom to know what we should pray for. Now I submit that far too often we are simply lazy. I tell you it grieves my soul that so many churches pray for one another in trite ways (bless those on the prayer list for you know their needs) for many secondary things (99% of all prayers are for physical needs). It is NOT wrong to pray for another’s physical needs, but are there no spiritual needs in the lives of God’s children? Have we all arrived and have no impatience or rebellion in our hearts? Do we not care enough for people we know who are suffering in myriad ways to actually go before God with their names on our lips? These are things about which we know to pray. The Spirit helps with what we do not know. And when we are truly at the end of ourselves and know not what to pray, then the Holy Spirit intercedes for us with unspoken groaning that we do not comprehend. There will always be times we know not what to pray for, we will always be dependent upon God for grace in this age. When are we not faced with the infinite distance between us and God?

This brother sheds more light on the topic. Very good read.