It's a free online image maker that allows you to add custom resizable text to images.
It operates in HTML5 (if supported), so images are created instantly and you can draw on them.
You can also add a scumbag hat or a custom image to the image you're captioning.
Most commonly, people use the generator to add text captions to established memes,
so technically it's more of a meme "captioner" than a meme maker.
However, you can also upload your own images as templates.