Lowest common denominator (often shortened to LCD) has two distinct meanings:

1. In mathematics, it's the smallest number that two or more fractions can share as their bottom number (denominator). When you need to add 1/4 and 1/6, you can't just add them directly because they have different denominators. The lowest common denominator is 12, because both 4 and 6 divide evenly into 12. You convert 1/4 to 3/12 and 1/6 to 2/12, and now you can add them: 5/12. Finding the LCD makes fraction problems much easier to solve.
2. More commonly in everyday language, it refers to the most basic level that appeals to everyone, especially when that means making things less thoughtful or less challenging. When a TV show aims for the lowest common denominator, it's trying to attract the biggest possible audience by keeping everything simple and obvious, avoiding anything challenging or sophisticated. A teacher frustrated with constant interruptions might complain that the class keeps sinking to the lowest common denominator instead of having thoughtful discussions. In this sense, the phrase suggests settling for what requires the least from everyone, rather than encouraging people to rise to a higher standard. It's usually not a compliment.