I've made this plot in Rstudio using the metafor plot:

enter image description here

As you can see, it's a meta-analysis forest plot with age as a moderator. I just don't know what the grey diamonds show and I've been scouring forums to find out. I know that it shows something about the moderation, but I don't know what the size of the diamonds mean. It's probably right in front of my face. Thank you!