Imagine that you are in space and you are the center of that infinite space.
All around you are spirals of energy. You could see them as sheets of ephemeral fabric. 
Those are the dimensions. They are all around you, or you could say that you are in the midst of them, all the time.
Now, within those dimensions are timelines. You can imagine them as threads through the fabric of the dimensions.
When you are feeling a certain way, say peaceful, you are in a vibratory pattern (or timeline) that exists in Peace.
When you are angry for example, you are in a vibratory pattern that contains the energy of anger.
There is no wrong or bad timeline, and anger (as one example) is not inherently bad, it is simply a feeling, and as a human, you chose to experience all of the feelings.
As you change the way you feel, multiple times a day, you are moving through the threads of timelines, as all of them are becoming available to you as you ascend.
The dimension that earth has existed in for eons has been the 3rd dimension, one of duality, and very limited movement.
As you are awakening, you are remembering that you can choose your state of being, therefore you can move within timelines. This is how you are moving out of the 3D existence and into the next dimension: 4D.
4D has its own characteristics.
It looks and feels much like 3D, but it is not the same. 
4D has many levels and layers, including levels in the spirit world.
This is how many of you are becoming aware of and more comfortable with your “psychic gifts.”  In 4D you are more conscious of your multidimensionality, meaning you are aware that you can connect to spirit, or other realms more easily. You are aware that you are manifesting your heart’s desire more easily because they already exist in the higher realms. You are recognizing that you are not nearly as limited and constricted as you used to believe you were.
You are moving between the 3rd and 4th dimensions on a regular basis and in many cases, reaching 5D awareness in your meditation, or when you practice yoga and other disciplines which bring the body, mind and spirit into deeper alignment.
Now 5D is very different, and you have glimpses of it, especially when you dream.
Play with this, allowing your expanding mind awareness to imagine being between dimensions, moving amongst timelines. The best way to begin is to be in your natural state of Being; in a state of peace and gratitude.