How to not conflict between policies on AWS?

I'm trying to make my AWS environment safer. So, I just created a bucket, no modifications at all, on S3, and want to give a minimal set of permission to my application users. So, I have created an user called keeper, he can, basically, only assume some role that I can specify in his policy. And this is working because I have assume a role and requested a temporary session token for him via CLI. However, in this role I have specified a policy that, is possible to upload a file to a specific path, in my new bucket.

However, this is not working, giving me permission denied. Do I need to specify a bucket policy where it says that it's possible to upload an object to my path ?

If so, what is the meaning of roles then ? I'm a bit confused about resources policy and users/roles/groups policies.