You can toggle Mark/Unmark Packet or Ignore/Unignore Packet from the Wireshark Edit menu, or by right-clicking on a packet in the Packet List pane and selecting Mark Packet (toggle) or Ignore Packet (toggle).

The menu displayed by right-clicking on a packet in the Packet List pane is shown in the following screenshot:

Wireshark allows you to mark one or more packets in the Packet List pane to make it easier to find those packets later by giving the packet entry a black background with white font. This marking can be toggled on and off on a per-packet basis. Marking a packet has no other effect on the display or packet context.

You can also ignore one or more packets. However, when you invoke the ignore function on a packet that packet entry disappears from the Packet List, Packet Details, and Packet Bytes panes and it effectively ceases (temporarily) to be part of the capture file. Note that ignoring packets can result in Wireshark reporting re-transmissions or other error conditions caused by the missing packet.

The ignored packets aren't actually deleted from the capture file as you can use the Reload option in the View menu or click the Reload icon on the icon bar to recover the ignored packets.