The contribution that Mendeleev is most well known for is his version of the periodic table and the contributions to what is called the periodic law. The periodic table that Mendeleev created was considered first because it was made available to the public first. That same table has evolved over the years to include new elements and updated data.
The periodic law, which is credited to both Mendeleev and Lothar Meyer in 1869 while working separately, states that properties repeat when arranged according to their mass. Using this information and the existing information of already known elements, Mendeleev was able to predict the existence of elements that had not been discovered at that time.