We all have negative things happen to us, and sometimes those things get stuck in our nervous systems or emotional centers in our brains. EMDR is a therapy that utilizes bilateral stimulation, which means we access both sides of the brain/body system with rapid eye movements back and forth, (or sometimes use tactile buzzing or sounds), to stimulate the whole brain to deal with the problem or issue that has been stuck. This is particularly helpful in traumatic experiences, but also very useful in any situations that feel unresolved in a person's life.
In the cases where EMDR is helpful, it's like digging out the dandelion roots in your lawn instead of just mowing over the top of them. When the emotional root of the problem is destroyed, your present is much more manageable.