A dacha is basically a country or out-of-the-way suburban home to which one goes for vacations. Out of the way from the cities, usually in a village near the sea or a river or a forest, for swimming, hunting, gardening, growing home produce and the like. Vacation house.
Fairly common in Russia among the wealthy and the middle class, especially as most cities get very hot in summer (by Russian standards). A poor family can also have a dacha inherited from older generations, often old and run-down, or a normal country house in a village in the middle of nowhere converted into a dacha. The Black Sea (that round thing bordered by Turkey, Romania and Bulgaria among others) is a popular location. A dacha is different from a normal country house in that it generally has no sources of heat, being a cheap structure that rides the line somewhere between log cabin and trailer, and it's too cold to live in it during the Russian winter. Legally, it could not be your registered place of residence, and was not considered a second home in the Soviet Union.
Also known as 'Datsche' in the former GDR.
Americans in certain regions may be familiar with the similar, though less common, practice of having a small "lake house" or "hunting cabin" on a small plot of land in an isolated area, often inherited from a time when fishing, trapping, and hunting for part of the year was a pragmatic way to supplement a family's food supply rather than a luxury.