los
go
As a command or signal to begin or hurry up.
Auf die Plätze, fertig, los!
On your marks, get set, go!

go / loose
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go
As a command or signal to begin or hurry up.
Auf die Plätze, fertig, los!
On your marks, get set, go!

'Los' is a very common separable prefix. In many verbs such as 'losgehen' (to set off/start), 'losfahren' (to set off by vehicle), or 'loslassen' (to let go), it expresses the beginning of an action or a separation.
Wann fahren wir endlich los?
When are we finally setting off?
As a single word, 'Los!' (Go!/Come on!) is a command or prompt to begin immediately or hurry up. It is very direct and often colloquial.
Los, wir müssen zum Bahnhof!
Come on, we have to get to the train station!
To ask what is happening, the question 'Was ist los?' (What's going on?) is extremely common. The answer can be 'Hier ist viel los' (there is a lot going on here) or 'Nichts ist los' (nothing is going on/it's quiet).
Was ist denn hier los? Warum ist es so laut?
What is going on here? Why is it so loud?
In the meaning 'locker' (loose) or 'nicht fest' (not fixed), 'los' almost always comes after the noun as part of the predicate. You say 'Der Haken ist los' (the hook is loose), but not 'der lose Haken' for this usage.
Achtung, die Schraube an deinem Fahrrad ist los.
Careful, the screw on your bicycle is loose.
The expression 'etwas los sein' (to be rid of something) means that you are free of something. The thing you no longer have is in the accusative.
Ich bin meine Kopfschmerzen endlich los.
I'm finally rid of my headache.
The expression 'etwas loswerden' (to get rid of something) describes the process of freeing yourself from something. The object you want to get rid of is in the accusative.
Wie kann ich diese alten Möbel am besten loswerden?
What is the best way for me to get rid of this old furniture?
fast / quick
'schnell' and 'los' can both appear in situations where someone is supposed to hurry. 'schnell' describes the speed of an action or says that something should happen quickly. 'los' here is more of a signal or command: someone should start or get moving. Learners may confuse the words because you often hear both together, for example 'Los, schnell!' (Come on, quickly!). On its own, however, 'los' does not simply mean 'quickly', but rather 'start' or 'go now'.

Los, zieh deine Jacke an, das Taxi wartet schon.
Come on, put on your jacket, the taxi is already waiting.
Wir müssen schnell gehen, sonst verpassen wir den Bus.
We have to walk quickly, or we'll miss the bus.