Transfer signifies the carryover of linguistic features from one language to another. The principle idea behind L1 transfer is that language learners are essentially summoning all of their linguistic resources in the L1 to help them deal with a gap in their knowledge of the target language. Sometimes L1 transfer results in grammatically correct communication (positive transfer) and sometimes in learner errors (negative transfer or interference). When positive transfer happens, an instructor or an addressee will probably not even realize it happened. On the other hand, interlanguage is often full of errors because of negative transfer. Such errors are both a common and natural part of language acquisition.
Bibliography
Scovel, Tom. Learning New Languages. Heinle & Heinle, 2001.