To append string2$ to string1$ means to join string2$ to the end of string1$.
To append to the end of a String you should (for maximum speed of code execution) use
X$(inf:0)="append this to end"
X$(inf:inf)="append this to the end"
Here, inf denotes infinity.
So X$(inf:inf) means "the substring of X$ starting at infinity". This is particularly useful when you don't know how long your string is and do not want to calculate its length.
see also: prepend