The Discernment of Spirits is all about growing in spiritual self-knowledge, learning how to defend yourself from the bad and grow towards the good. It is a way of interpreting the movements of your soul, noticing everything from emotions to desires and learning to understand where they come from and where they might lead you. St Ignatius referred to these as ‘the motions of the soul’.
Essentially, St Ignatius believed that your soul is affected by a ‘good spirit’ and a ‘bad spirit’. He taught his followers how to recognise each spirit and therefore live better lives, not pushed around by their temptations and worst inclinations but instead guided by their best qualities. In this way, the discernment of spirits can help you to grow in self-knowledge and move towards becoming the best version of yourself.
Talk of good and bad spirits can easily make the modern person uneasy or sceptical, but a universal part of the human experience is inner conflict. People are often drawn away from life-giving things of God into their personal weaknesses and bad habits. A universal part of the human experience is inner-conflict. St Paul described this experience in this way:
‘I do not understand my own actions. For I do not do what I want, but I do the very thing I hate.’ Romans 7:15
The conflicting voices affect even saints like St Paul! But what’s going on here? It is certainly the case that people are often drawn away from the life-giving things of God into their personal weaknesses and bad habits. St Ignatius believed that the bad spirit was the influence of the devil on our soul, and Pope Francis shares his view, saying:
‘We should not think of the devil as a myth, a representation, a symbol, a figure of speech or an idea. This mistake would lead us to let down our guard, to grow careless and end up more vulnerable.’
Discernment of Spirits can help you to recognise this ahead of time and to more quickly stop this downward momentum, allowing yourself to be drawn back towards God.
St Ignatius describes the behaviours of the different spirits in his rules for the Discerning of Spirits.
The Good Spirit
‘... give courage and strength, consolations, tears, inspirations and quiet, making things easy and removing all obstacles, so that the person may move forwards in doing good.’ St Ignatius of Loyola, Spiritual Exercises 
For anyone who is genuinely seeking to do what is right, the good spirit will bring enthusiasm, life, strength, tears, encouragement, views of a way ahead, peace, and consolation. The good spirit also brings realism, an acknowledgement and sorrow for sin, but peace as a loved and forgiven sinner called to follow Christ
The Bad Spirit
‘... it is typical of the bad spirit to harass, sadden and obstruct, and to disturb the soul with false reasoning, so as to impede progress’. St Ignatius of Loyola, Spiritual Exercises 
The bad spirit will bring endless problems, difficulties, doubts, desolation, and confusion. It can bring about feelings of hopelessness, being weighed down with no taste for prayer or spiritual things - even encouragement to give up!
St Ignatius, a former soldier, uses a military example to illustrate the behaviour of the evil spirit, likening it to a hostile army commander who is strategic and precise:
‘ … he behaves like a military leader setting about the conquest … the enemy of our human nature makes his rounds to inspect all our virtues … and where he finds us weakest … there it is that he attacks and tries to take us.’ St Ignatius, Spiritual Exercises 
God encourages and beckons, gently inviting us on. The bad spirit forces and drives, suggesting there is no choice.
Ultimately, discernment takes effort and practice. All of these principles and insights from St Ignatius help, but true Discernment of Spirits requires real maturity and an ability to observe God at work in your inner life.