# Why can the L1 Norm be expressed as a constrain?

I am learning why the L1 regulariser (Lasso) is used to encourage sparsity in ML models. When describing the proof, I am seeing that the regularised minimisation cost function;

$$min(RSS(w) + \lambda* \| w\| )$$

can be rewritten as a constrained but smooth objective;

$$min(RSS(w))\;\;\;\;\; s.t. \; \| w\| \leq B$$

and that would mean we are optimising on the L1 unit circle.

I was hoping someone could help me understand why we can write the L1 as a constraint and what does that mean?