“Prior to that, I spent two years as an administrative assistant at a recruiting agency.”

You're in a job interview. You're describing your work history. You started by explaining your current job. You say this to explain the job you had before that one.

Prior to that, I spent two years as an administrative assistant at a recruiting agency.

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prior to (something)

"Prior to" means "before". But it sounds more formal. People use "prior to" in business situations, in a courtroom, and so on. There are some other differences between "prior to" and "before as well. You can say:

Before you go, can you take a quick look at this email I'm sending to Tracy?

But you can't replace "before" with "prior to" in that sentence. You have to follow "prior to" with a noun:

Please call to confirm your flight details prior to departure.

"Prior to departure" means "before you leave". The phrase "prior to ___" can also include a verb with the "-ing" ending:

Prior to meeting Vick, I had never even seen an opera, much less known an actual opera performer.

spend (time) as

You use the word "spend" with an amount of time because we think of time as being very valuable, just like money. To "spend time ___ing" means to use your time to do it:

We spent the first month just debating what to call ourselves.

People use the phrase "spend (time) as ___" to express how long they did a certain job for. You use this phrase when you're describing your past jobs, but usually not to describe your current job.

administrative assistant

An "administrative assistant" used to be called a "secretary". In recent years, the word "secretary" isn't used much because it seems old-fashioned and sexist (meaning that it seems disrespectful to women). An administrative assistant helps busy and important people in a company to schedule their meetings, send letters and packages, and other things.

a recruiting agency

A "recruiting agency" is a company that finds employees for other companies. If you need to find a certain kind of employee, but it takes too long to find people like that, you can hire a recruiter to find the right person for the job. Recruiters will often call people who are already working at other companies, and will try to convince those people to change their jobs.

There's also another word for recruiting agents. People call them "headhunters". "Headhunter" is also a word to describe members of native tribes in places like South America who kill their enemies and take their heads. So, as you might expect, the word "headhunter" is not a very positive way to refer to a recruiter!