Why do I need to pass value for Serial type in Insert/Update?

I don't know if jdbc/java code requires this, but when I use two gui admin tools I found, and I insert a row into the table using their row editor feature, both require me to enter a number for the Serial type. I thought this type was used to auto-increment an id field and that I would not need to enter anything into it? Basically we need the normal indexed ID field for each table, and we want it to auto-increment. The serial shows a function of nextVal() or something like that, so I assume it auto-increments, and it shows unique and not-null. Can someone explain how serial is used, why would I still need to pass a value for it?

Thanks.

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