What is "udev" and "tmpfs"

udev and tmpfs in the output of df command refer to filesystem types. You're probably seeing something like this:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            3.9G     0  3.9G   0% /dev
tmpfs           787M  1.5M  786M   1% /run
/dev/sda1        28G   25G  1.6G  94% /
tmpfs           3.9G  193M  3.7G   5% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
/dev/sdb1       110G   81G   24G  78% /mnt/extra
tmpfs           787M   40K  787M   1% /run/user/1000

tmpfs - essentially is a virtual filesystem located in RAM instead of a disk device. Since it's a filesystem, data saved there has a certain order, just like a regular filesystem for a disk storage would have, however the files reside in memory and are not persistent (that is, those files will be gone next time you power off the computer - and it's OK, that information is necessary only for the duration of the system running and no reason to store data on disk). In some other Linux distributions you might see /tmp directory serve as a mountpoint for one such tmpfs filesystem.

udev is also a filesystem, which is also virtual, however there's many more pieces to the overall udev system. Information stored in this filesystem is of course related to the devices files - aka the interface between actual physical device and the user. You can read more about it on a related question. Interesting behavior of this filesystem is that data doesn't really accumulate in certain files when you write to them - this is different compared to regular disk filesystems. For instance, consider the character device /dev/null or /dev/tty1.

As for "What should I do with them?" question, the answer is "nothing". For a casual user they're not interesting. They run from RAM, they don't eat up actual disk space, and they play somewhat important purpose in the system. Software developers, sysadmins, and advanced users - they'll have a good reason to create another tmpfs for their purpose or they'll have a reason to poke around /dev or modify the configuration/rules for how udev treats newly added devices to the system. But of course - those types of users do have a reason to "do something" about these filesystems

Note that although /dev/sda1 appears in Filesystem column, it is actually a device file. What actually is on that device represented by /dev/sda1 might be ext4 or NTFS filesystem, and you can see that with lsblk -f or df -T command.