By is a preposition. It often means at, by, near, or sometimes with, depending on context.
In this sentence, by die stasie means at the station.
Some examples:
So by is a flexible word, and here at is the best translation.
Die is the Afrikaans definite article, meaning the.
One nice feature of Afrikaans is that die does not change for gender the way it does in some other languages. It is used very widely for the, whether the noun is singular or plural.
So die stasie simply means the station.
This is the normal word order for a simple main clause in Afrikaans:
A very important rule in Afrikaans is that in a main clause, the finite verb usually comes second.
So:
If you move another element to the front for emphasis, the verb still stays in second position:
That version is more marked or emphatic, but it still follows the same basic rule.
It usually means at the station, but the exact physical meaning can depend on context.
It could mean:
If you specifically want to say inside the station, you would more likely say:
So by die stasie is best understood as at the station unless the context says otherwise.
A rough pronunciation guide is:
Ons is al by die stasie
≈ ons iss ahl bay dee STAH-see
A few notes:
This is only an approximation, but it is close enough to help an English speaker get started.