"Resolver" parameter defines the location of DNS server that nginx must use in order to resolve the IP of the URL passed under proxy_pass;

As explained by Tarun, by default nginx will pick your resolver from the host /etc/resolv.conf and once resolved, it will cache the IP. Resolver is mostly used in two cases:
1. Either in a private network, to resolve the IP's that exist in your network.
2. Or used at a place where the IP of your proxy_pass or upstream location changes very frequently and you cannot rely upon nginx cached IP.

In the example you specified, the resolver will be the IP of the DNS server that could resolve your location. This could be either of :

1) 127.0.0.1 : If the web server itself is a DNS server, for this you need to setup DNS server on port 53(default) of this server.

2) x.x.x.x : The IP of the DNS server hosted in, either in your private network or any public DNS server if your URL's are publicly accessible. One may use 8.8.8.8 (Google's public DNS server).

3) You specified 10.x.x.x : Assuming that you were referring the AWS documentation. If not, in general, 10.x.x.x again needs to be a DNS server IP, which in case of AWS is 10.0.0.2. AWS reserves a few IP's of its VPC's and the second IP x.x.x.2 is reserved for DNS server. Note that in case your VPC is not 10.0.0.0/16, this IP will change accordingly. Eg: Lets say your VPC is 10.192.0.0/16, then you will be using 10.192.0.2 as resolver.

For above ref to https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html

https://www.jethrocarr.com/2013/11/02/nginx-reverse-proxies-and-dns-resolution/