Tree particles fail

Hello! I'm new in blender and I was trying to put trees in a forest scene I was doing, I'm actually following this tutorial: youtube.com/watch

each time I try to put some trees in the landscape with the particle system, they look like that.

I found a comment for solving this problem, but I don't understand what he means with the "modifier>convert":

"For anyone who can't get their leaves to appear at 13:00, Click on the tree. Go to Modifiers > Convert. Select the trunk and all the branches > Ctrl+J to join. Go to the landscape mesh. Remove the tree collection from the particle system and re add it. The origin for the tree may change so make sure it's correct :)"