Metta Meditation is meant to cultivate more Loving Kindness towards yourself and others. Loving-kindness is a central virtue of Buddhism and Metta Meditation is away to develop this virtue. If Practised together with Vipassana Meditation it is a very powerful Meditation.
The mind is just like a muscle and needs to be trained in order to perform well. With Metta Meditation more Loving Kindness is cultivated which enables you to share love whole-heartedly. The meditator first focusses on sending Loving Kindness to himself or herself.
If you experience resistance it indicates that feelings of unworthiness are present. The practice is here to help you overcome these negative feelings.
Once resolved and the feeling of love towards oneself is sufficiently experienced the Loving Kindness Meditation can be focussed towards loved ones a next step would be to include neutral persons and eventually even hostile persons and be able to feel Compassion towards them as well.
Metta Meditation needs to be practiced. If you are eager to understand how to perform the Meditation please follow this link where the technique is explained in a very clear manner. Or check Zen Buddha's Meditation videos.