Locking in the Web Client
When locking is enabled, objects are read-only by default. There is no button to save edits to the current object. You must explicitly select the button to lock an object and begin editing.
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Object Page > Toolbar > button

When this button is selected, the object is locked and the and buttons are now shown.

When another user opens a locked object, it is read-only and cannot be copied.
The button is displayed for the user who locked the record if that user has at least level 3 (Read/Copy) security on the object.