What does Memory Warning mean for a plugin?
When you install a custom plugin it allows you to be able to run custom commands that can potentially use a lot of memory. For example the WorldEdit plugin allows you to generate massive Minecraft structures by entering commands into the console.
Whilst plugins are incredibly powerful, it can lead to maxing out your servers memory and crashing your server. So this Memory Warning message is a friendly reminder to be careful when issuing commands, as typing 1000 instead of 100 could crash your server.
For example, the WorldEdit //hsphere command will build a sphere structure, in the below example with a 30 block radius.
//hsphere -r diamond_block 30
However, if you try and build a sphere with a 1000 block radius, there's a good chance the server will run out of memory.
//hsphere -r diamond_block 1000
So please be extra careful when issuing commands while running these plugins!