Filtered aggregates are used to include only some rows into the aggregation

select customer_id, 
       sum(amount) as total_amount,
       sum(amount) filter (where amount > 10000) as large_orders_amount
from customers
group by customer_id;

This can be re-written as

select customer_id, 
       sum(amount) as total_amount,
       sum(case when amount > 10000 then amount end) as large_orders_amount
from customers
group by customer_id;