How difficult is it to refinish a hardwood floor? What is all involved

If the flooring is indeed real hardwood, then although not impossible, yes it is difficult. All the finish must first be completely removed. This will likely involve renting machines made for this purpose, and the physical ability of someone capable of managing them.

If the flooring is not solid hardwood, but only a top finish, and if it is too fargone to live with, it may be cheaper labour-wise to have it taken out, and new flooring installed.

I would watch some videos on YouTube, watch the sanding, sometimes filling, staining and then poly finishing of floors.

I think they key phrase in Flipturn7's answer is "the physical ability of someone capable of managing them". If you don't know how to use the heavy machine you could make the floor totally uneven. You could try YouTube, as Sharon suggested, but remember, most of these people are pro's and know what they are doing. If it's real hardwood, you don't want to ruin it. You might see if you could find a professional to sand the floor then stain and seal it yourself. That might be a good compromise. Think about it and think about your skill level. It's totally up to you.

First you must sand. Renting a sander and learning how to use it can be a nightmare. You can gouge the' floor, so make sure you watch some DIY videos about it. After the first sanding with a heavy grit then we do with a light sand. I always like 3 coats of varnish to finish it off. I have had a pro do the sanding.

Lots of sanding then refinish with a good sealer It is work but worth it.

I, too, recommend getting a professional to do the sanding as the crazy, heavy sander can take you on a ride you may never forget! Staining, etc. is not impossible but is time-consuming as is the sealing with multiple coats needed. May I suggest you practice staining and sealing to get the hang of the rhythm. Reread Flipturn and the others' suggestions, watch some videos and practice before making your final decision!

it can be time consuming you need to rent or buy a big sander and then seal the floor

You are welcome. Real hardwood is not that common these days. Properly refinished, it can give many more decades of service. The cost of labour divided over years ahead will likely be worth it to have the job done by someone with the experience and big tools.

You have approached it the right way- to start asking questions and get info first before just diving into it.