In math, an open sentence, also called open statement, is a sentence that is neither true nor false. Basically, just looking at the sentence, you cannot tell if it is true or false.
For example, 1 - x = 8 is an open sentence. The reason 1 - x = 8 is an open sentence is because we do not know the value of x just yet and as a result we cannot tell if it is true or false.
This uncertainty that we have about 1 - x = 8 is what makes 1 - x = 8 an open statement.
More examples of open sentences
4x = 20
6x - 4 = 32
31 - n = 27
40 ÷ n = 5
e2x = 4