What are Governor Limits in Salesforce? Can you name 3 examples?

Adding some points:

In a SOQL query with parent-child relationship sub-queries, each parent-child relationship counts as an additional query. These types of queries have a limit of three times the number for top-level queries.

The row counts from these relationship queries contribute to the row counts of the overall code execution.

In addition to static SOQL statements, calls to the following methods count against the number of SOQL statements issued in a request.

Database.countQuery
Database.getQueryLocator
Database.query
From above statement i can clarify one point:

For example your org contain  only 2 Account and each have 10 Contacts.

If you execute this with sub Query like below

Select Id, Name, (Select Id, Name from Contacts) From Account

Then the total number of Query rows will be 22 (2+10+10).

From above analogy we can understand like this

If your org contain 40M accounts and each have 1 contact.

Then in this scenario you can use sub query up to 25M only.

Like this Select Id, Name, (Select Id, Name from Contacts) From Account limit 25M

Thanks