We recall first the construction of the analytification of $X$. Let $U={\rm Spec}(A)$ be an open affine subset of $X$, then ${U^{\rm an}}$ is the set of multiplicative seminorms on $A$ extending the given absolute value $|\phantom{a}|$ on $K$. This set is endowed with the topology generated by the functions ${U^{\rm an}}\rightarrow{\mathbb{R}},p\mapsto p(a)$ with $a$ ranging over $A$. By glueing, we get a topological space ${X^{\rm an}}$ which is connected locally compact and Hausdorff. We can endow it with a sheaf of analytic functions leading to a Berkovich analytic space over $K$ which we call the analytification of $X$. For a morphism $\varphi:Y\rightarrow X$ of algebraic varieties over $K$, we get an analytic morphism $\varphi^{\rm an}:{Y^{\rm an}}\rightarrow{X^{\rm an}}$ induced by composing the multiplicative semiorms with $\varphi^{\sharp}$ on suitable affine open subsets. We refer to [Be90] for details, or to [BPS11], §1.2, for a neat description of the analytification.