A collision is when a file in the source and destination differ, yet have the same name. That is, the same file (path\name) is both in the source and destination but the two copies differ in some way, perhaps by date, size etc. It is not an error, but a normal situation to encounter, and one that SyncBack is designed to help you resolve.
A notification of collisions occur in the Differences window, which appears by default when making a backup (note that there are circumstances when the Differences window does not appear, for example when the user has chosen not to show the window).
Collisions are shown in red in the Differences window to highlight there are going to be changes made when you continue the profile task. If the user views the Differences window carefully, the user has the option to make choices about whether they want to accept the changes SyncBack is proposing, or bypass them by right-clicking and choosing a different Action from the pop-up menu. The standard Windows mass-selection and/or 'cherry-picking' options are available to enable selection of multiple files before collectively changing the Action for all the selected files (to the same new Action) by right-clicking any of the multiple selected files.
Note: the right-click 'pop-up' (context) menu Actions are automatically limited to those appropriate for the situation. For example, you cannot choose the Action 'Copy to Source' if the file only exists on Source in the first place. This can mean that multiple selections are limited to those Actions appropriate to all the selected files, so that (using the same example as the previous sentence), if you accidentally selected such a file, that Action would be unavailable for the whole of the multiple selection even though it is appropriate for most selected files (the point being that it is not appropriate for all of them). The solution is to identify and de-select the file(s) for which the desired Action is not appropriate or possible.