SSH access is the way to access your server where AEM is deployed. It is a secure shell mechanism to access your hosting server. AMS will give you details of IP and credential and you can connect to the server via SSH to access your logs and repository.
And also can anyone tell me about Segments in AEM?
to understand that the repository has 2 components i.e NodeStore and DataStore.Oak Segment Tar is an implementation of NodeStore. It is an Oak storage backend that stores content as various types of records within larger segments. Segments themselves are collected within tar files along with further auxiliary information. For complete information, kindly refer https://jackrabbit.apache.org/oak/docs/nodestore/segment/overview.html#