Tidally Locked Planets (or any tidally locked celestial body) is an object whose rate of rotation matches the orbital period around another object. As such, the same side of the orbiting body always faces the center object (though there may be some slight wiggle). Our own moon (while obviously not a planet), experiences such a tidal lock on Earth, and thus the same side of moon is always facing Earth while it’s rear always faces away.

Planets tidally locked to a star will have one side of their surface experience perpetual daytime while the other experiences perpetual night. There may be extreme temperature differences between the opposite sides of the planets surface, though this may not always be the case. Both air and sea currents will be impacted.

Worldbuilders wishing to generate believable ecosystems for tidally locked planets should consider how life may have evolved or adapted to live on one side of the planet or the other.