“They're not actively searching for a replacement yet.”

You're quitting your job at the end of this year. Your company hasn't started interviewing people to fill the job yet. You're talking to a friend who asked you about the job. You say:

They're not actively searching for a replacement yet.

(someone) is actively (doing something)

Doing something "actively" means that you're working on it and directly trying to do it. In the example above, the company isn't trying hard to find a new employee yet.

The opposite of "actively" is "passively". To understand the difference, think about two ways that you can search for a new job:

• actively searching for a new job means responding to job postings, sending your résumé to lots of companies, attending networking events, and so on
• passively searching for a new job means doing something like posting your résumé on an Internet job board, and waiting for interesting companies to contact you

search for a replacement

When someone leaves a job or gets let go, you have to "search for a replacement". This means interviewing and hiring a new person to take the job.

You can call the new person who takes a job a "replacement" for the old person. A "replacement" is permanent. If an employee is gone temporarily but is going to come back later (like because of a pregnancy), you hire a "fill-in" or a "temp".