A LOG field, when added to a layer, allows your crews to keep a running history of maintenance or other notes associated with individual features. A LOG field displays on a layer's feature form just like any other field but with a button titled "Expand". When you click this button the historic log is expanded to full screen where you can scroll through the past notes or add a new note. Any newly added notes are automatically stamped with the date and time and your email address. By design, previously entered notes cannot be altered by anyone. This gives you confidence in the integrity of the log. If an error is made on an entry then you can submit a new note describing the correction.
Don't confuse the LOG field type with the LOGSHEET field type. Both are used to keep a running list of historic information. Where the LOG field type is used for narrative history, the LOGSHEET field type is used for tabular readings. For more about both of these field types see Field Properties.
See the non-editor exception settings on the Layer Settings screen which allows users who don't have edit permission on a layer to still be able to log notes.