For the word expansionism, think of something expanding, of getting bigger. This time, it's not your waistline, it's a country. Expansionism is how the United States ended up with 50 states instead of 13 colonies.
Way back when, expansionism used to be called "imperialism." But then imperialism got a bad name, so now it is called expansionism. Basically, it's a country trying to exert influence over another country, or actually moving on in or invading. Needless to say, the other country isn't always so happy about being swept up in expansionism.
