This is a very casual way of asking someone to sit down. 

After a long hard week, I’m really enjoying my free weekend when I can take a load off and watch some old movies. 

Here are a few examples:

You’ve been working all day, why don’t you take a load off and have a drink.

Grab a chair, take a load off. stay awhile.

Take a load off and tell me what you’ve been doing these days.

A variation is also  used to say “Don’t worry.”

Take a load off your mind.

Look at these test results, this should take a load off your mind.

I hope you were able to take a load off and enjoy a relaxing weekend. Now are you ready to start a new week?

Here’s a great song about “taking a load off”, from 1968, The Band singing The Weight.