Administrators are able to 'masquerade' as another user - that is to see the LMS as they do and perform actions as them. This can be done without needing to know their username and password.
This can be useful for troubleshooting issues, as well as performing tasks on behalf of someone.
Logging in as another user
First you need to locate a user's profile and select it
From the Settings block, select Site administration > Users > Accounts > Browse list of users
Filtering user list
To find a particular learner, or set of learners you must use the filters to narrow the list.
Filtering for Scripted Enrolments and Reports is explained here, and follows the same basic principle.
Selecting a user
Once you can see the learner you are looking for, select their name to view their profile.
Login as (and log out again)
From the Settings menu, select Login as
You will get confirmation that you are now logged in as them and will then be returned to the home page.
The login details at the top-right of the screen will update to reflect this as well. Once you are finished you can either select Log Out or just click your own name