A filter can be compared to a single condition in an if-statement. Usually (almost always, see Group) the operator in the if-statement is "&&" (in JavaScript/C#)/"AND" (SQL).

For example, the filters:

results in the if-statement (pseudo code):

If(asset.asset_type is in the supplied list of asset types 
	AND asset.assetid is in the supplied list of asset IDs
	AND asset.itemid is in the supplied list of item IDs) {
         AddAssetToResults()
}

A filter can be created by selecting the Input tab in the Search Editor and then clicking the "Add" button (see Create a Keywords filter for a detailed explanation on how to create a filter):

In the window which pops up, there are three tabs: Metadata field, Table and column, and Predefined. (For an image please see "3 Comparison Type")

A Metadata field or Table and column filter MUST have some properties defined before it can be added to a search:

The data, which the filter does work with. This is either a combination of Table and column or a Metadata field.
The Comparison Type, which decides what logic the filter will apply to the data being worked with.
The Input Type, which determines how the Media Manager and Video Portal gather input for a search (some Comparison Types does not have an Input Type associated with it; in these cases an input type is not required).
The ID.

If a predefined filter is needed, then the input from the user is usually only an ID. Though some predefined filters MAY have Predetermined values, which MUST be supplied before the filter can be created.