The 55mm is probably the lens focal length. The filter size should be on the front of the lens following a symbol that looks like a circle with a line through it
The camera body has nothing to do with the size filter you need nor does the focal length of the lens. The only thing that matters is the diameter of the front of the lens. USUALLY somewhere on the front of the lens (either around the front element) you'll find the focal length (the lens on my camera shows 67mm with a 0 with a line through it). It uses 67mm filters. If you're not sure what to look for, ask at a local camera shop or check online. I just looked at B&H Photo's website. Under Specs it listed 67mm as the filter size. Worst case scenario, ask here on the Hog. Make sure you include the manufacturer & focal length of the lens.
I have a Nikon D5600 and it says 55mm. But when I recently purchased some 55mm filters (UV, polarizing, etc), they do not fit. What am I doing wrong?
You apparently have a 18-55mm lens. If its "AF-P" type the 55mm filter is correct. If its "AF-S" type then you need a 52mm filter size. You can easily confirm this my looking at the back side of your lens cap, the proper filter size will be embossed there.
I have a Nikon D5600 and it says 55mm. But when I recently purchased some 55mm filters (UV, polarizing, etc), they do not fit. What am I doing wrong?
If your D5600 came kitted with the AF-P DX NIKKOR 18-55mm, it would see from online sources this lens uses a 55mm filter. But, you didn't say which lens. Use the graphic below for where to look at the lens in question and confirm the filter size needed for your specific the lens. As Bob noted, the lens cap should also indicate the filter size needed.