What is meant by Adams Bashforth being a "boot strap" method? People seem to say that the Adams-Bashforth method requires some "boot strapping" because it needs two initial conditions: $y_{n+1}=y_n+\frac{\Delta t}{2}[3f(t_n,y_n) - f(t_{n-1}, y_{n-1})]$ I understand that, if only one initial condition is provided, then another initial condition must be derived. So, are we saying that it is "boot strapping" because we have to just use one initial condition to get another "initial condition"? For example, using just regular Euler's or something to get another "initial condition" using the first one? If not, what is meant by "boot strapping" here? Thanks.

Your differential equation initial value problem is given as $y'=f(t,y)$, $y(t_0)=y_0$.

For the multistep method the first computable value is $y_2$. However, to compute that you also need $y_1$ which can not be computed via the multistep method and thus needs some other method to provide that value.

Note that if the multi-step method has error order $p$, then the initialization method should have at least error order $p-1$ so that the propagation of these initial local errors does not contribute a worse order term to the global error.