What does RRQMS and WRQMS mean? I found a possiblity to display Rrqms and Wrqms graphs in AWS RDS Enhanced monitoring page (I use Postgres 9.5.19) but I can't find any detailed explenation about these parameters.

By googling I only found these two very vague explenations:

From http://heshen.lofter.com/post/cfa6d_b1b5cb0


rrqms: The number of read requests merged per second that were issued to the device.
wrqms: The number of write requests merged per second that were issued to the device.


From https://www.codetd.com/en/article/7988617


rrqms: the device associated with the read requests per second is the number of the Merge (when the system call requires the read data, VFS requests sent to each FS, FS if found different read requests to read is the same as the Block data, FS will request the merge combined)
wrqm / s: the device associated with the write requests per second is the number of the Merge.


These explenations though are still very blurry for me. Yes one is read and the other is about write per (milli)second but merged into where? Came from where? Issued by whom etc...
RDS instances use Elastic Block Storage (EBS). EBS documentation explains what request merge means:

the underlying drive technology determines the maximum amount of data that a volume type counts as a single I/O. I/O size is capped at 256 KiB for SSD volumes and 1,024 KiB for HDD volumes because SSD volumes handle small or random I/O much more efficiently than HDD volumes.
When small I/O operations are physically contiguous, Amazon EBS attempts to merge them into a single I/O operation up to the maximum size. For example, for SSD volumes, a single 1,024 KiB I/O operation counts as 4 operations (1,024÷256=4), while 8 contiguous I/O operations at 32 KiB each count as 1 operation (8×32=256).

High number of merged I/O requests means your database is requesting a lot of sequential blocks, and EBS is able to translate that into fewer but larger, hence more efficient I/O operations. Low number of merged I/O requests probably means your database is mostly accessing random blocks.
To answer your

Came from where? Issued by whom?

-- by whatever is reading from and writing to EBS volume(s) on your RDS instance, presumably Postgres.