Making the "Assumption" when calculating equilibrium

[William Lan 2l]

So, I know that you assume x to be negligible when it's less than 5% of initial value. But what is the initial value? Is the initial value the initial concentration of the product/reactant? And how would you know x is less than 5% of initial value if you don't even know what x is?

[Ashley Chipoletti 1I]

I believe in one of the videos on his website Dr. Lavelle said that a when the K value is less than 10^-4, you can make the "assumption".