"Trendy" means "fashionable" or "in style".
But "trendy" can be used either positively or negatively. Here's a positive example:
That's a nice jacket. It would look great paired with a trendy top.
When you call something "trendy" negatively, it means that something is fashionable in a very temporary way, that's going to change soon. For example:
I can't waste time paying attention to what's trendy!
You can call clothes and other fashion "trendy". You can also use "trendy" to describe things that are recently popular like restaurants, diets, specific political opinions, slang phrases, websites, neighborhoods, etc.
My brother dragged me out to this trendy nightclub that just opened up downtown.