If you use quadrant based house systems, the IC and MC will be the divisions that create the 4th and 10th houses. They're significant because they're part of the cardinal points in the natal chart, meaning that anything located or associated with it will have an increased amount of significance to the native. The effects of (a planet in) the cardinal houses are very forceful and noticeable.
These houses were also called pivots or hinges because they represent the height or a high point in the chart. If you follow the diurnal motion of the sky, it goes in clockwise position. A planet in a succedent house can be likened as a person climbing to reach to the peak, which are the cardinal points. Once these planets reach the peak, they are heightened and elevated. Then after the peak, they fall or decline, which are symbolized by the cadent houses; planets here represent a low energy state, and are weaker as a result.
The IC represents the ancestry, roots, grandparents, the father (or mother, depending on which system you follow); it rules the land, anything underground, the home, and the emotions surrounding it. It represents the foundations of our life. The IC was called the subterraneous place by the ancient Greeks. Since it is the lowest point of the Earth (opposite of the midheaven, the highest), it can also represent things that end. It also represents hidden things, as the MC represents the most public and outward things.
The MC represents the reputation, advancement, and occupation. Glory, honors, authority, popularity, prestige, and success. This house is the highest and most visible point of the chart, as it represents the highest point of the Sun's travel. This house represents our desires to achieve.
Any planets here color the meanings of the houses, and the sign that the cusps are on will color how the matters are manifested in the native's life. Planets that are conjunct the angle will be very noticeable.
While they may not be as significant as the chart ruler, they're up there of high importance.
If you want to start to interpret your house meaning, check to see if there are planets there. If not, then check to see where the rulers are and their conditions.
