In a one period model replication means that no matter which state of the model it holds that
a S_1+ b B_1 = D_1
where $a,b$ are the quanties of stock and bond held and $S_1,B_1$ and $F_1$ are the prices of the stock, the bond and the derivative at $t=1$. In such a case the fair price of the derivative at time $0$ is $a S_0 + b B_0$ (not thinking about correct discounting). Fair here means that it can be replicated by other assets in the market (in the given model).
For multi-period settings usually one defines the notion of a strategy to be self-financing. Thus you shift between the stock and the bond but all money is either borrowed from/put on the money market account or financed by sales of the stock. Self-financing means that there is no money coming from outside.