Q: What is SMR?
A: SMR is short of "Shingled magnetic recording" is a magnetic storage data recording technology used in hard disk drives to increase storage density and overall per-drive storage capacity. Shingled recording writes new tracks that overlap part of the previously written magnetic track, leaving the previous track narrower and allowing for higher track density.
Q: Why should I care if a drive is SMR?
A: In some circumstances SMR drives are much slower than conventional CMR drives and they are more suitable for use in "write once read many times" scenarios like archiving, movie storage etc. than in multiple small writes scenarios like large databases. Also it's never a good idea to mix CMR and SMR drives in the same RAID array.
Q: Where can I read more about SMR and CMR (and PMR)?
A: You can read more about SMR on Wikipedia. Also Synology website has a very good explanation. However be aware that there is a terminology confusion and some sites refer to CMR as PMR (first CMR disks were called PMR but when the SMR disks were invented, they had to be renamed to CMR - see https://blag.nullteilerfrei.de/2018/05/31/pmr-smr-cmr-i-just-want-a-hdd-mr/). PMR stands for Perpendicular Magnetic Recording and both SMR and CMR are based on PMR. So if an manufacturer specifies that a drive is PMR that doesn't automatically mean that it is CMR, it can be a SMR as well!
Q: How can I test if a drive is SMR?
A: You can check the answer to this particular question here https://unix.stackexchange.com/questions/286432/how-to-determine-whether-hard-drive-uses-smr.