That entry is a PHP serialized array. The serialize method takes an array and stores it’s value as a string so it can be stored elsewhere – like a database.
In the above example, the “s:” entries stand for “string:”. With a serialized array, each element’s data type is represented by a letter (a, s, o, etc) followed by a colon, then an optional length, and then a value. So…
a:5:{i:0;s:35:”add-from-server/add-from-server.php”
a:5:{... - array, 5 elements
i:0 - integer, value of 0
s:35:"add-from-server/add-from-server.php" - a string that is 35 characters long, and then the string itself
Edit: by the way, you should always interact with these by using the PHP methods to serialize and unserialize – if you removed the “.php” from that string above, you would break serialization unless you also changed the length in “s:35” to “s:31” (since four characters were removed). It’s easy to break a WordPress installation by hand-editing serialized arrays.