Join in yarn MC? Help!

Can anyone clear up what the line “join in yarn MC” means in the following instructions for me? This is at the very beginning of a pattern for a dress:

“Using size 8 needles and waste yarn, cast on 14 sts. Beg with a K row, work in St st for 4 rows, ending with RS facing for next row. Break off waste yarn and join in yarn MC.”

This is the first time I’ve used an invisible cast-on, and I was able to do that just fine. But what does it mean to join in the main colour yarn, though?? And how? Help!

Up until now you’ve been working with your waste yarn. Once those 4 rows are finished, you will just start knitting with your main colour. Insert your needle into the first stitch (waste yarn stitch) and use the working end of your main colour to knit it and then continue on. It will seem loose at the beginning of that row but you can weave that end in later OR you can knit the first few stitches of the main colour with both the working end and the tail end of your main yarn and then the tail will be knitted in and won’t have to be woven in later. Good luck!