English has main clauses and sub-clauses.
Main clauses are usually the main part of a sentence.
They can be a sentence by themselves.
I like ice cream.
Sub-clauses add more information to the main clause.
They cannot be sentences by themselves.
I like ice cream because it is delicious.
Putting the sub-clause first
A lot of students learn English structures as starting with a main clause (independent clause i.e. something that can be a sentence by itself), then adding sub-clauses.
However, it is also possible to put the sub-clause first. It is not as common as putting the main clause first, but it is important in adding variation.
Dave cooked the lamb because he didn’t like chicken.
= Because he didn’t like chicken, Dave cooked the lamb.
If the sub-clause is first, it is usually followed by a comma. However, it is not a strict rule.
Examples
Because there were no seats left on the plane, they had to take the bus.
Since she was five years old, Anne has enjoyed table football.
In order to get there on time, the boys ran.
For the final two years of his life Dave’s father lived as a recluse.