To struggle means to wrestle with some kind of difficulty. It can refer to internal or mental turmoil or an actual physical encounter or combat. It can be used as a noun or a verb. Examples: 1) John struggled with his weight for many years. 2) Mary, who has dyslexia, often struggles to comprehend what she is reading. 3) The struggle over Crimea may never end.
In the US today, this word is normally used in only a few situations, and almost always as a verb - this is a serious word and so it doesn't come up often in everyday conversation and writing. The following are some of the usual usages of the word - you don't hear it often outside of such situations: