Full spectrum diplomacy is when a country uses all of the tools it has to make friends and solve problems with other countries. Just like how you might use different tools in your toolbox to build something, a country has different tools it can use to build relationships with other countries.
One tool might be to talk with leaders from other countries and try to find common ground. Another tool could be to send aid to a country that is struggling. Yet another tool might be to let people from other countries come visit and learn more about your country.
Using all of these tools together is like making a cake with all of the ingredients. Each ingredient adds something special to the cake and makes it taste better. In the same way, using all of the different tools of diplomacy makes a country's relationships with other countries stronger and more successful.