The Testimony is also referred to as the Ark of the Testimony, but more commonly referred to as the Ark of the Covenant.
So, why is it called the Testimony? The Ark of the Testimony is a container, one that is to place things inside of it. There were only key things placed inside the Ark. Those things which were a testimony to the things G-d has done for the Israelites. These things were special things between G-d and the Israelites. They were things to help them remember what G-d has done for them.
Do we have something like this in our life? Are we keeping an account of the things G-d has done for us? It is our testimony. Each of us has a testimony with G-d and it is the multiple stories of where G-d has brought us from, and out of, to where He has taken us.
It is good for us to remember these details. This is why G-d had them build the Ark of the Testimony. So that they would remember where G-d brought them from and what He did for them.
We all should have some special record safely kept for recording those things which G-d does for us and with us. As we go through life, we will have an account of the glorious things we have seen through G-d. It will remind us when we doubt or are struggling. It will serve as an inspiration for hope.
G-d is awesome and He does wonderful things in our lives. Sometimes we need a place to go to remember those things that G-d did personally for us. Reading about the Ark of the Testimony, I am inspired to keep a record of what I remember G-d doing for me personally, then continue to write things down as they happen.