When a planet is "tidally locked", it is similar to the relation the Earth and Moon share. It means that the same side of the planet is facing the star, so if Earth were tidally locked, there would be no days, no rotation, just a world that would be half day and half night... forever.
This particular phenomenon creates an interesting mixture in exoplanet Upsilon Andromeda b. The planet is tidally locked with its parent star, so one half of the planet is hotter than lava; the other half less than freezing. The temperature range of this exoplanet is astonishing: from -4 to more than 3000 degrees Fahrenheit.