How did you end up setting up your grow tent?
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To answer your original question: An inline fan is one that’s designed to be installed “in line” with the ducting of a ventilation system. A 6" inline fan, for example, has inlet and outlet ports that are sized so that 6" hoses can be clamped to them.
That makes an inline fan a waste of electricity.
Mostly wasted because your tent is open and the garage door is partly open. Do you know what PPFD your light can deliver? To get a noticeable benefit from added CO2, you need more PPFD than the light in a typical kit can produce.
Why is your tent open? Ordinarily, they’re kept closed (along with any ports that lack screens) to exclude unwelcome insects.
Do you have any plants?
@Venturi all understood, my inline fan is working great.
My tent is closed but I do keep 2 flaps open (flaps with mesh to protect against insects). Garage door is slightly open to increase co2 but is closed about 12 out of 24 hrs. No complaints when it comes to the light, PPFD reading is perfect for what I have growing and at different growth stages. My co2 reading though is between 420-450 on average, sometimes going up to 500. I think I would like to improve that in the future. That’s why I asked about the inline fan, to get more info on what else besides the obvious it does.
Thank you for you reply 
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I only have one hose clamped on and exiting the tent. I don’t see how more could be attached. It’s a SpiderFarmer 2024 design, 2x4 tent.
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Oh I see what you mean! One side out of the tent and the other connected to the filter via a smaller hose clamped together.
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Good. I thought you meant the main ones, which puzzled me. You were referring to your air-inlet ports.
Obviously, those ports must be open for your exhaust fan to work properly. @PhotoFinisH (who I believe is our resident expert on airflow and circulation) has explained already how to adjust them so you get adequate airflow (CFM) and good airspeed at the inlets.
The inline (exhaust) fan helps control temperature and humidity, draws CO2 in, and contributes to circulation. Maybe that’s all obvious.
Right. I connect them with a short piece of rubber hose. Once installed, its rigidity makes it easier to move the fan and filter as one unit.
A carbon filter is needed only if you need to control the smell and, even then, can be omitted until you start flowering. Unless the filter is much larger than is usual for your exhaust fan’s size, it will reduce the fan’s maximum CFM substantially.
That’s great info, really appreciate it. Is co2 of 420 ppm enough do you think or should I work at increasing it?
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Given that atmospheric CO2 is roughly 416 ppm these days, 420 ppm isn’t going to help much.
The figure below is taken from Photone’s website. I don’t know the original source.
They go on to recommend 400 - 600 ppm for seedlings and clones, 400 - 1200 ppm for vegetative, and 800 - 1500 for flowering.
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Thank you, great info. I’m looking at increasing co2.
Happy growing!
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The source for the figure I posted appears to be this Fluence Photobiology Guide. It includes the figure and table below – I find the figure more informative than Photone’s version. (The entire guide is worth reading, BTW.)
Notice how the benefit of added CO2 increases as PPFD increases.