The teraflop, or TFLOP, is essentially a mathematical measurement of a computer's power. Without going too fancy, a teraflop translates to a computer being able to process one trillion calculations a second. Xbox Series X has 12 teraflops, meaning it can handle up to 12 trillion operations a second. What makes teraflops a useful measurement for computing power is their inclusion of floating-point numbers.
Floating-point numbers basically covers all types of numbers, and not just integers (whole numbers). In other words, numbers with decimal points, irrational numbers, and any other irregular figures are accounted for when talking about floating-point calculations. Because computers (and game consoles) use a lot of non-integer numbers when processing, the teraflop is a more accurate measurement of a device's capabilities.
More teraflops means more complex calculations per second, which roughly translates to a faster machine with more graphical grunt. For the record, Xbox Series X's 12 teraflops is a lot.