I can't call other people's code from Lisp. This is mostly untrue. Most Lisp environments give you a way to call external routines using either C or Pascal calling conventions. You can also call back into Lisp from the external program. Lisp's garbage collector causes unpredictable pauses when my program runs.
Lisp programs create garbage by destroying all references to some object in memory.
In a program written in some other language, the programmer must arrange to release the memory occupied by the object at the same time when the last reference is destroyed. If you've ever written a complex program that allocates and manually recycles a lot of dynamic memory, you know how difficult a problem this can be. Lisp finesses the memory leakage problem by never allowing the programmer to release unused memory. The idea here is that the computer can determine when a block of memory is unreachable with complete accuracy. This unreachable block is said to be garbage because it is no longer useful to any part of the program.