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Albedo is a measure of how much sunlight is scattered back away from the planet. If you decrease the albedo, more sunlight reaches the surface and the surface, hence the atmosphere warms. Warming could result in more heat being trapped by the planet's atmosphere as the surface tries to radiate the heat into space (assuming the atmosphere has substantial water vapor and CO2 in it) which could provide a green house effect. Whether that becomes "run away" is subject to a number of variables.

A planet's albedo can change due to natural cycles or disasters. For instance, if the re were lots of volcanic activity throwing ash high into the atmosphere, the albedo could decrease and remain that way until the ash settled out of the atmosphere.

Albedo is a measure of reflectivity from 0 to 1. Generally, whatever light is not reflected back is absorbed and converted into heat instead. The higher the albedo, the more light is refected away and so the less warm it becomes. Have you noticed how something that is dark colored will heat up faster in the sun? It has a lower abledo than something which is bright colored.

The temperature to increase. As the surface darkens more of the sun's energy would be absorbed and less reflected back out into space.

Ice melting will decrease the albedo.