A matrix $A=[a_{ij}]\in\mathbb{F}^{n\times n}$ is called upper triangular if $a_{ij}=0$ whenever $j<i$. If we write this matrix out it looks like $$A:= \begin{bmatrix} a_{1,1} & a_{1,2} & a_{1,3} &\cdots & a_{1,n} \\ 0 & a_{2,2} & a_{2,3} & \cdots & a_{2,n} \\ 0 & 0 & a_{3,3} & \cdots & a_{3,n} \\\vdots & \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & 0 & \cdots & a_{n,n}\\ \end{bmatrix}$$