There are some bugs that can lead to accidently sending plaintext. It is recommended to use PGP or OMEMO instead. See OMEMO.
The plugin won't work with Gajim >= 1.0 as it was not ported to python3. There is no port planned.
This plugin provides experimental, potentially insecure support for the chat protocol indepentent OTR encryption.
OTR stands for Off-the-Record and is designed to provide encryption, authentication, deniability and perfect forward secrecy. See the creators' website otr.cypherpunks.ca for more information on the protocol.