
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!

going on
Describes that something is in progress or happening. Often in the question 'Was ist los?' (What's going on?).
Auf der Straße ist heute aber viel los.
There is really a lot going on in the street today.

loose
Not fixed, loose, or detached.
Der Knopf an meiner Jacke ist los.
The button on my jacket is loose.

to be rid of
To no longer have something or someone; to be free of something.
Endlich bin ich meine Erkältung los.
I'm finally rid of my cold.

to get rid of
To free oneself from or part with something or someone, often also in the sense of selling something.
Ich möchte mein altes Fahrrad loswerden.
I want to get rid of my old bicycle.
'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?
now
In a command, jetzt emphasizes that something should happen immediately. Los particularly urges someone to begin the action or hurry up. Both can be used with the same action, but they emphasize different aspects.

Los, räum dein Zimmer auf!
Come on, clean your room!
Räum jetzt dein Zimmer auf!
Clean your room now!