If $\le$ is a partial order on a set $A$, two elements $a,b$ of $A$ are comparable if either $a\le b$ or $b\le a$; otherwise they are not. For instance, in the partial order $\subseteq$ on $\wp(\Bbb N)$ the sets $\{0,2\}$ and $\{0,1\}$ are not comparable: neither is a subset of the other. On the other hand, the sets $\{0,2\}$ and $\{0,1,2\}$ are comparable: $\{0,2\}\subseteq\{0,1,2\}$.
Another name for a total order is linear order. It expresses the intuitive idea very well: you can picture a linear order $\le$ on a set $A$ as arranging the elements of $A$ in a line. The usual order $\le$ on $\Bbb R$ is a linear (or total) order: if $x,y\in\Bbb R$ are any real numbers, either $x\le y$, or $y\le x$. To put it another way, if $x$ and $y$ are any real numbers, at least one of the statements $x\le y$ and $x\ge y$ must be true. Contrast that with subsets of $\Bbb N$. If $A$ and $B$ are subsets of $\Bbb N$, it’s not the case that at least one of $A\subseteq B$ and $A\supseteq B$ must be true: we just saw that $A=\{0,2\}$ and $B=\{0,1\}$ are a counterexample. The linear order $\le$ lines up the real numbers as a single linear arrangement; the partial order $\subseteq$ does not line of the subsets of $\Bbb N$ in a linear arrangement.