The syntax is (for filter):
[the table to search].filter(filter expression).[value to return]
The filter expression may vary depending on what you are looking for but usually it’s something like [the column to search in = the value to search for].
The filter formula (without the [value to return] part) returns a list of rows. The [value to return] specifies what to do with the data, eg. - get specific column, sum, etc.
If you want more specific help for your need, please embed an example doc here so we can see what you are looking for.
See also Here