Astronomy:

The wanderers, better known as the seven wanderers, is a group of objects visible in the night sky of northern Westeros, and probably the rest of Earth as well. They are brighter on some nights than others. One of the wanderers is red.

While there are seven "stars" which are identified as the wanderers, comets have also been referred to as "wanderers."

The seven wanderers are referred to as stars, but they are presumably analogous to the planets of our own Solar System. Additionally, the word "planet" is derived from a Greek word meaning "wanderer." Of note is that in A Song of Ice and Fire the seven wanderers are spotted with relative ease by those familiar with the night sky, whereas we can only see six of our planets with the naked eye, and one of those (Uranus) was not identified as a planet until the 1700s.

Culture:

The seven wanderers are sacred to the Faith of the Seven. The Septons preach that the red wanderer is sacred to the Smith.

Maester Luwin taught Jon Snow how to find the seven wanderers in the night sky. Therefore this is presumably a common part of education for the Westerosi highborn.

Recent events:

When the Red Comet appears in the sky over Westeros, it is referred to as a wanderer.

One clear night beyond the Wall, Jon Snow spots the seven wanderers.