How does Google Analytics credit a channel that contributes to a conversion prior to a transaction?
Assist interaction is any interaction that is on the conversion path but is not the last interaction. If a channel appears anywhere—except as the final interaction—on a conversion path, it is considered an assist for that conversion. The higher these numbers, the more important the assist role of the channel.
Read more here: https://support.google.com/analytics/answer/1191204