Why is tRNA called an adaptor molecule?
RNA and proteins are not electrical systems, but the idea of translating a signal between incompatible systems can be applied here. 

Proteins are made through a process called Translation, where instructions stored in a piece of mRNA are used by the Ribosome to make protein.

tRNAs adapt the mRNA nucleotide sequence into a protein's peptide sequence by acting as an intermediary. A tRNA is charged by attaching an amino acid to one end, it then binds the ribosome mRNA complex at the position defined by a codon. A codon is a sequence of 3 nucleotides that codes for an amino acid. 



You could think of the tRNA being like a SD Card to USB adaptor to get information from one device to another.