Light doesn't always travel in straight lines. It bends and changes direction for a number of reasons. The most common reason is refraction when light travels in transparent media of different density, as in lenses, or in the atmosphere, or as it passes from water to air.
The effect of curved light paths is that the object you see is not actually where you think it is. This makes spearing fish in shallow water very difficult, as the fish is not where it looks like it is.
Gravity also bends light, but the effect is negligible in normal life.
Black Holes are collapsed stars with gravity so strong that light they emit curves back into them, so you cannot see them - hence the name.