What is a serviceaccount in kubernetes?

Question

I am a newbee in Kubernetes and I am not able to understand the following things.

1) We have multiple nodes in a cluster. When we create a user(serviceaccount) does it mean we refer to a node? If not, who exactly is a user in a cluster?

2) Can you explain what a context is in simple terms and how is it related to cluster, namespace and users?