We are rebranding our business and our design team suggests to go for a standard typography style. So we are looking for best and affordable fonts and stumbled across Google Fonts. Those are very good options and we need to choose Open sans & Rubik for our Brand guidelines book. Is this a good choice? It says open source license, what does it mean?
There is nothing wrong with using Google Fonts in your branding. Of course it would be ideal to support independent type foundries and purchase directly from the creators, but sometimes that's not an option. There are a ton of high quality fonts available on there, and they're all free to use for personal or commercial projects, in print or on the web. Honestly, using Google Fonts may be one of the easiest ways to implement brand consistency across mediums.
If you want other options:
Creative Market has a ton of fonts you can buy for cheap (like $5-$25 cheap). They're boutiquey though, so your fonts may not be organized as nicely, or you may run into some performance issues.
But if Google Fonts offers what you need, go for it. Just know you aren't getting a unique / custom font for your brand. Google Fonts are widely used so odds are a ton of people are already using it. And if you're super happy with it, maybe kick the creator a few bucks for their work :)