Yes when you download 1 GB of data such as a 1 GB movie or 1 GB other file you use up 1 GB of data to download that amount.
So if you download a lot of 1 GB size files or watch a lot of movies then you could go through a limited data plan pretty quickly.
Every time you view websites, send emails, watch videos etc you use up data.
With 1 GB of data you should be able to send at least 1,000 emails which is a lot before you run out of your data.
Or you should also be able to view 1,000 websites or web pages depending on the size of the website page file.
If the website has more images and other heavy files then you may only be able to view 500 websites or web pages with 1 GB of data.