Consider the set $\mathbb{R}×\mathbb{R}$ . Denote the general element of $\mathbb{R}×\mathbb{R}$ by $x×y$ . The set $\mathbb{R}×\mathbb{R}$ has neither a largest element nor the smallest element , so the order topology on $\mathbb{R}×\mathbb{R}$ has as basis the collection of all open intervals of the form $ (a×b,c×d)$for $a<c$ ,and for $a=c$ and $b<d$ . The sub-collection consisting of only intervals of the second type is also a basis for the topology on $\mathbb{R}×\mathbb{R}$ ..
Doubt
I am unable to understand the highlighted part. Also usually figures are associated to show intervals . I am not able to understand those figures( figure below)
NB- Just started studying a month ago from Munkres. This is from section 14 from his book. 
The order topology is a type of topology used on product sets. The example used is the order topology on $\mathbb R\times\mathbb R$. The order topology is different from the usual topology on $\mathbb R\times \mathbb R$. The order topology is so named because it comes from an ordering placed on the underlying set. For $\mathbb R\times\mathbb R$, a point $a\times b$ is less than $c\times d$ if either $a<c$ or $a=c$ and $b<d$. This is sometimes called the dictionary order because it resembles the ordering of words in a dictionary: first you compare the first component, and if the first components are equal, you compare the second coordinates.
Just as a basis of the regular topology on $\mathbb R$ is the set of all open intervals, the basis of the order topology on $\mathbb R\times\mathbb R$ is also the set of all open intervals. So what is an open interval in the order topology? The open interval with endpoints $a\times b$ and $c\times d$ is $$I=\{x\times y\in\mathbb R\times\mathbb R:a\times b<x\times y<c\times d\text{ in the dictionary order}\}.$$ So what does such a set look like? There are three possibilities. If $a\times b\geq c\times d$ in the dictionary order, then the set $I$ is empty (this possibility is not pictured in Munkres). If $a=c$, then $I$ is a vertical line segment from the points $a\times b$ and $c\times d$ (excluding the endpoints). This is the second of Munkres' two pictures. If $a<c$, then $I$ is the vertical ray starting at $(a,b)$ and continuing upward, all vertical lines with $x$-coordinate between $a$ and $c$, and the vertical ray terminating at $c\times d$. This is the first of Munkres' two pictures.