The /use line has two pairs of brackets with conditions inside, either of which must fully be met for the spell to cast.
It always starts with the first set of brackets:
'@focus' sets the target of that spell to your focus, this is unconditional.
'exists' and 'nodead' check both that your focus (the new target of your spell) exists and is alive: If either are false, the first conditional (set of brackets) will fail and be ignored.
If both are true then the conditional will succeed and the spell will be used, at your focus.
So you can read it as "If my focus exists and is alive, cast at them; if not,"
Now, if the first set of brackets fails, it moves to the next pair.
The second pair are empty, meaning they will always be true unconditionally so the spell is used at your normal target.
The conditioned target will be your default target: It's thus equivalent to [@target].
You can simply read it as "cast at my normal target".
Putting the two meanings together:
If my focus exists and is alive, cast at them; if not, cast at my normal target.
Finally, SpellName is the spell that will be cast.
You can put multiple spells onto a single line by seperating them by semicolons, an example is here:
/use [help]Heavy Windwool Bandage;[@focus,harm][harm]Throw
If I can help my target, use a bandage on them, else; if I can harm my focus, throw a knife at them, else; if I can harm my target, throw a knife at them.
One final example is modifiers: You can change which conditionals succeed by holding any modifier, the most convenient of which are shift, control and alt.
If I'm holding a modifier (shift, control or alt) then kick my target, else; gouge them.
If holding shift, cast kick; if holding alt, cast gauge; else cast throw.