What is the meaning of read -r?

There is no stand-alone read command: instead, it is a shell built-in, and as such is documented in the man page for bash:

read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

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-r
Backslash does not act as an escape character.  The backslash is considered to be part of the line.  In particular, a backslash-newline pair may not be used as a line continuation.

So, to summarize, read normally allows long lines to be broken using a trailing backslash character, and normally reconstructs such lines. This slightly surprising behavior can be deactivated using -r.

The -r option prevents backslash escapes from being interpreted. Here's an example:

Assume there's a file with this content:

ngRTM6hNqgziZcqCcEJN7bHAP9a1GeMs\
Ni3EAX1qvogWpRIPE3oagJL6nwl\QQW9y
bjJHyaVBrUcyZOY5U4h9QHnpEPqg\\\\\\\\\Q9Fk
iNOvAyBTAcN5n1uwR4GvRfAGUbPWiXax\n
cqGPPStH3gaWolrfVAlMtoWiSuLa7GzQ\n\n\n
EnO04N1nEkpWbfXRxrtYNqCZDpF\trQIXS

$ while read line; do echo $line; done < tempfile
ngRTM6hNqgziZcqCcEJN7bHAP9a1GeMsNi3EAX1qvogWpRIPE3oagJL6nwlQQW9y
bjJHyaVBrUcyZOY5U4h9QHnpEPqg\\\\Q9Fk
iNOvAyBTAcN5n1uwR4GvRfAGUbPWiXaxn
cqGPPStH3gaWolrfVAlMtoWiSuLa7GzQnnn
EnO04N1nEkpWbfXRxrtYNqCZDpFtrQIXS

$ while read -r line; do echo $line; done < tempfile
ngRTM6hNqgziZcqCcEJN7bHAP9a1GeMs\
Ni3EAX1qvogWpRIPE3oagJL6nwl\QQW9y
bjJHyaVBrUcyZOY5U4h9QHnpEPqg\\\\\\\\\Q9Fk
iNOvAyBTAcN5n1uwR4GvRfAGUbPWiXax\n
cqGPPStH3gaWolrfVAlMtoWiSuLa7GzQ\n\n\n
EnO04N1nEkpWbfXRxrtYNqCZDpF\trQIXS

The Bash man page's section about read states that, by default...

The backslash character (\) may be used to remove any special meaning for the next character read and for line continuation.

but, if you pass -r, then

Backslash does not act as an escape character.  The backslash is considered to be part of the line.  In particular, a backslash-newline pair may not then be used as a line continuation.

A little thought suggests that the only possible "special meaning" they could be talking about is the character serving as a delimiter. And sure enough, without -r, you can backslash-escape a delimiter or a newline, but with -r, you can't, and backslashes just get interpreted as literal backslashes:

$ read -d 'x'    var1 <<< 'There was once \
a curious Uni\x user.xHe did a little test.'
$ echo "$var1"
There was once a curious Unix user.
$ read -d 'x' -r var2 <<< 'There was once \
a curious Uni\x user.xHe did a little test.'
$ echo "$var2"
There was once \
a curious Uni\