I’m using US terms.

what you can do with it:

Clusters are the same as working 3 (or more) stitches together, so it’s basically a decrease.  You can make these in almost any stitch because the premise is the same — we’ll be leaving off the finishing loop of three stitches until the end of the cluster and close them together at the end.

how to do it:

My examples are using double crochet.  Chain the appropriate amount for what you want to do (I did multiples of 3+4).  In the 5th chain from the hook, work a dc stitch, but don’t work the last loop.  Work 2 more unfinished dcs in the next 2 sts, so that you have a total of 4 loops on your hook:

Yarn over, pull through all 4 loops.  You’ve made the cluster stitch:

If you’re not wanting to use this as a decrease stitch, make sure to chain the appropriate number of stitches between clusters.  Since I’m doing my clusters right next to each other, I needed to chain 2 in between my cluster stitches.