Just has several meanings.

Time

When used with a verb, just indicates the immediate past.

They have just gone. (= They went a very short time ago.)

She just phoned. (= She phoned a very short time ago.)

‘Where is the file?’ ‘I’m just bringing it.’ (= immediately)

I saw him just last week. (= I saw him as recently as last week.)

Just now can mean ‘at the moment’ or ‘a few seconds ago’.

She is not in just now. (= She is not in at the moment.)

I saw James just now. (= I saw James a few seconds ago.)

I am busy just now. (= I am busy at the moment.)

Only, scarcely

Just can mean ‘only’.

I just want some rest.

I just want a place to sit down.

I just want something to drink.

I just want somebody to understand me.

I just want a good job.

Just can also mean scarcely or almost not. This meaning is often emphasized with only.

We only just caught the train.

Exactly

Just can mean exactly.

It’s just 2 o’clock. (= It is exactly 2 o’clock.)

That’s just what I meant. (= That is exactly what I meant.)

Just as an emphasizer

Just can be used to emphasize other words and expressions. It means ‘simply’.

You are just beautiful.

I just love the way she smiles.

Tenses

When just means a moment ago, it is usually used with the present perfect tense in British English. In American English, a simple past tense is more common.

They have just arrived. (GB)

They just arrived. (US)