When viewing a record detail page (e.g. Account detail) the related lists shows a limited number of related records at a time, e.g. for Opportunity related list. So, if you have more than 5 (by default) opportunities tagged to an account, in the Account detail page, you will see You will see Show x more » | Go to list (y+) ». By default x will be 5 and y is total number of the records.
You can scroll down to the bottom of page, and look for Always show me more records per related list or Always show me fewer / more records per related list if you have click more before (even in previous login).
Clicking more or fewer increases and decreases the default number of related list records displayed for all object record detail pages for the logged in user. If you notice, when you click more or fewer link, in the URL, salesforce will add parameters rowsperlist=xx, for example: https://cs30.salesforce.com/0018000000Ril4o?rowsperlist=10, this parameter will show related list up to 10 records for ALL related list.
Please note: this change will be permanent for the logged in user for ALL tab.
How is this different from the "Show more" link on the a related list?
The more / fewer link at the bottom of a record detail page is not the same as the Show more link on the bottom of individual related list sections on the record detail page. The "Show more" link increases the number of records displayed for that related list only (not all related lists on the page) for one time only.
If the user views another record and returns to the previous record, they will have to click the "Show more" link again in order to increase the number of related list records for the related list they wish to view.