An access port hands a device plain, untagged Ethernet frames and quietly does the VLAN bookkeeping itself. A trunk port carries several VLANs over one wire at once — and the only way to keep them straight is to tag every frame.
Every switch port has to be configured one of two fundamental ways, and the choice isn't cosmetic — it determines what kind of Ethernet frames that port is willing to send and receive. This explainer builds directly on the companion piece on broadcast vs. collision domains — VLANs are exactly the tool that segments one large broadcast domain into several smaller ones, and access ports and trunk ports are how a switch actually attaches devices to, and carries traffic between, those VLANs.