What is sense and antisense RNA?

Usually sense is a concept used to compare the polarity of nucleic acid molecules, such as DNA or RNA in molecular biology.

Antisense RNA is a single-stranded RNA that is complementary to a messenger RNA (mRNA) strand transcribed within a cell. It is used in silencing of mRNA. Since it is complementary to mRNA, it can bind to mRNA and form double stranded RNA molecule that do not translate. Antisense RNA strand has the sequence similar to Antisense DNA strand or template strand  on which the actual synthesis of RNA takes place, except for the uracil residues.

 Sense  RNA is a term that can be referrred to mRNA molecule synthesed. It has the sequence similar to sense or coding strand of a DNA molecule except for the uracil residues.