@InvocableMethod annotation helps to invoke apex class and method from the Salesforce Agentforce Apex Action.

@InvocableVariable annotation can be used to pass multiple parameters from the Salesforce Agent Action to the Apex Class.

Sample Apex Class:

public class AgentforceUtility {
    
    public class PicklistInput {
        
        @InvocableVariable( required = true )
        public String objectName;
        
        @InvocableVariable( required = true )
        public String fieldName;
        
    }
    
    @InvocableMethod(
        label='Fetch Picklist Values' 
        description='Fetching Picklist values for the object and field passed'
    )
    public static List < List < String > > fetchPicklistValues(
        List < PicklistInput > picklistInputs
    ) {
        
        PicklistInput objPicklistInput = picklistInputs.get( 0 );
        String strObjectName = objPicklistInput.objectName;
        String strFieldName = objPicklistInput.fieldName;  
        SObjectType objType 
            = Schema.getGlobalDescribe().get( strObjectName );
        Map < String, Schema.SObjectField > mapFields 
            = objType.getDescribe().fields.getMap();
        SObjectField objField = mapFields.get( strFieldName );
            Schema.DescribeFieldResult fieldResult = objField.getDescribe();
        List < Schema.PicklistEntry > picklistEntries = fieldResult.getPicklistValues();  
        List < String > tempProdFamilyPicklistValues = new List < String >();
        List < List < String > > productFamilyPicklistValues = new List < List < String > >(); 
        
        for ( Schema.PicklistEntry objEntry : picklistEntries ) { 
           
            tempProdFamilyPicklistValues.add(
            	objEntry.getValue()
            );
            
        }
        
        productFamilyPicklistValues.add(
            tempProdFamilyPicklistValues
        );
        
        return productFamilyPicklistValues;
        
    }
    
}

Agentforce Apex Agent Action Configuration:

Apex Test class:

@isTest
private class TestAgentforceUtility {
    
    static testMethod void testFetchPicklist() {
        
        AgentforceUtility.PicklistInput objInput
            = new AgentforceUtility.PicklistInput();
        objInput.objectName = 'Product2';
        objInput.fieldName = 'Family';
        AgentforceUtility.fetchPicklistValues(
            new List < AgentforceUtility.PicklistInput > {
                objInput
            }
        );
        
    }

}