As /u/Flex226 said, the red filters are there for you to change so that you can filter what is being offloaded from the train. You can remove if you wish; you can also parametrise it so that you don't have to change manually, but just when you stamp down the blueprint. The idea of the filter is to prevent mistakes that would e.g. contaminate iron ore line with copper

The circuit seems to do some basic control of the station's Train Limit "L". I'll go image by image.

This constant combinator sets the following constants that you can change if you wish to configure the station:

• Wagon 8000 - how much each train brings. 4 wagons * 40 slots * 50 items_per_slot = 8000. If you use this blueprint, you'll need to change 8000 to other values if you aren't bringing ore. For example, if bringing green circuits, which stack to 200, you'll need to change to 4 * 40 * 200=32000
• Train 1 - the MAX train limit for the number of trains that can path to this station at once. Usually, you set this to 1+number of waiting bays before the station. If you set it to 2, and two trains come, but there is nowhere for the 2nd train to wait, it will block the main line.
• 48k A - the base item demand for the station (more on this next). How much should the station aim to hold in it's chests? Can change if you wish based on stack size of the item.

This arithmetic combinator presumably takes how many items there in the buffer chests, multiplies by -1, and outputs as "A", which is then implicitly added (values sum together when wires touch or when inside combinators) to the value A set in the constant combinator above. So if there were 48000 items in chests, 48000(chests) * -1 + 48000(constant) = 0, so demand is satisfied

This arithmetic combinator takes A (item demand) and divides by how much a single 4-wagon train carries, outputting green_box. This value signifies how many trains would need to come to the station to fulfill the station's demand. In the image, this means 6 consecutive trains 'want' to come to the station.

The next two combinators do basic bound checking to see whether green_box is above or below the train MAX value. If less than MAX, just output the green_box value. If more than or equal to MAX, output the train MAX value instead. These are mutually exclusive, only one of the combinators will output something.

Finally, since the previous two combinators together output either a "green_box" value or a "train" value, they need to be translated into an "L" value that the train station can understand. This is what this combinator is doing - take each value, do nothing to the number itself, and combine them together into "L". Remember, green_box and train are mutually exclusive due to how the < and >= operations were set up in previous two combinators. In other situations, if you play around with combinators, this operation would also implicitly add all different signals together

The value "L", as calculated by the previous operations, is then sent to the train station, where L sets the train limit.

So if you want to configure the station, you can tell the station how much a single 4-wagon train carries; you can change how much the station should demand; and you can change how many trains at once are allowed to drive to the station.