What does it mean if u get an interview for a job before the deadline for applications has passed?

You've got time to read up on the job.
They must be keen to interview you. Good luck 🤞🏻

They are interested in you, and they need to fill the position quickly.

That they are interested in talking to you

They're staying on top of it

depending on the sector, id expect it means you were in early and they are working through the people they want to talk to

They have got heaps of applications or they think you are to good to lose to another company