Literally nobody with knowledge worth sharing is going to reply to this, surely.
Did you not read their website?
A quick google gives: https://www.mi5.gov.uk/careers/i-can-at-mi5
"What does an admin officer do?
You’ll do lots of different things as you’ll work across a variety of departments. Each one offers a range of roles, diverse challenges and interesting insights. Day-to-day, you’ll be organising meetings, managing information and records, booking travel, looking after visitors and providing essential support to our intelligence officers. It’s all about helping us to run smoothly."
etc etc
Taken with the job description, the salary range and common sense, you could work out the basics?
But yeah, you've probably failed the first hurdle by posting here. Presume you also missed the quite clear instructions on their website about having to go through vetting:
"You should not discuss your application other than with your partner or a close family member, providing they are British. You should inform them of the importance of discretion. You should not post on social media about your application or discuss it with anyone else at this stage."