From the Cisco switch manual that I have (Cisco SG-300):
The Port VLAN Membership page displays all ports on the device along with a list
of VLANs to which each port belongs.
As it says - this page displays all the ports and lists what VLANs they are members of. On this page you can see overall picture of VLAN assignment.
Use the Port to VLAN page to display and configure the ports within a specific
VLAN.
Here you choose a VLAN that you want to configure and it will let you to specify settings for every port in regards to the particular VLAN that you wish to configure. For example if you are configuring VLAN 5 than this page will display settings for all ports in regards to VLAN 5 only.
You can use both ways to properly configure ports in regards to what VLANs they participate to.
As you say in your comment
One is port to vlan and the other is vlan to port. I don't understand
why there are two. They seem to be the same.
They allow you to achieve same results in slightly different way.
Port to Vlan
andVlan to Port
pages look like?