Wave a flag is a simple challenge you can find on the picoGym.

The hint is as follows

Can you invoke help flags for a tool or binary? This program has extraordinarily helpful information.

This tells us that the challenge revolves around an executable file and we want to manipulate it somehow.

Before we can start manipulating it, we need to first download the program:
wget https://mercury.picoctf.net/static/f95b1ee9f29d631d99073e34703a2826/warm

Before we want to execute this file, we want to make sure the file we downloaded is executable
if [[ -x "warm" ]]; then echo "File 'warm' is executable"; fi

Next we need to make the file execute and chmod +x warm
./warm

This produces the following output:
Hello user! Pass me a -h to learn what I can do!

This is a -h flag, something that quite a lot of programs use to show a help list.

For example:

node -h

Once we run ./warm -h, We get our flag: