Please explain this

Shell Programming and Scripting
1 
cat $BDDATA/svucon$i_bal_dup_email.dat ; uuencode $dup_in $dup_out

here svucon$i_bal_dup_email.dat ???
and uuencode ??

2

what is the function of this code? explain background of this script? where and on which file did you apply this?

3

There are several problems with the line posted. We really need to see it in the context of the whole script and with a description of what the script is meant to do.

To answer one of your questions.
If there is a variable called $i which needs to be substituted into svucon$i_bal_dup_email.dat it will not work. This is because underscore is a valid character in a variable name.

Here is an example of what can happen. Note the use of braces to show shell where the variable name ends.

i=abc
echo First attempt  : $i_string
echo Second attempt : ${i}_string

First attempt :
Second attempt : abc_string

On the second point. The semi-colon is just allowing you to place two unrelated commands on the same line. It may be wrong.
The second command converts a file $dup_in to "uuencode" format outputting to $dup_out . There is no input from the preceding command.
The normal use of "uuencode" is to prepare a file to be an email attachment.