Access Request allow users to request access to content or a Site which they don’t have permissions to access by default. Once a Request is made, it comes under the Access Requests and Invitations group. The Site Collection Administrator or the Site Owner can either Approve or Reject the Request.
This option is located in Site Settings and under Users and Permissions. Pending Invitations are also listed. If an Admin sends out an Invite to an External User and it is not yet accepted, it gets listed under Pending Invitations.