I am reading a paper now and it is stated that some function $\phi$ can be defined as convex combinations of $q$ vertices $\theta$. According to this equation:
$$\phi(x, u, w) = \displaystyle\sum_{l=1}^q \mu_l(w) \phi(x, u, \theta_l) \\ \text{where:} \;\;\> \mu_l(w)\geq 0 \\ \text{and:} \;\;\> \displaystyle\sum_{l=1}^q \mu_l(w) =1.$$
I look forward to just simply understand this in a very basic way, this is just side note in the paper.