Complex sentence: Someone who is extremely "frum" or devout is known as a "Frummer" and it is used in a positive sense if he/she is "frum" unto them self but doesn't force it on others.
Split into simpler sentences: Someone who is extremely "frum" or devout is known as a "Frummer". This term is generally used in a positive sense if he/she is "frum" "unto themselves" but does not force their practice on others.