The correct answer to this open question is the following.
Although you did not attach the link to the video, we can answer in the following general terms.
What scientists mean when they said that ice cores are like tree rings is that scientists can determine the way the environment on planet earth has changed. For this to happen, scientists and researchers have to dig very deep into the ice shell to study the concentration of gases in the atmosphere and the changes with the pass of time. This is valuable information for scientists in order to know how the climate on planet earth has changed through history. And we are not talking about just 50 or 70 years. NO, we are talking about that the results of this kind of study allow scientists to have information of hundreds and thousands of years ago.