In architecture, a frieze refers to any horizonal panel used for decorative purposes. Triglyphs are panels with three vertical lines, and represent the beams used in prehistoric buildings to support the roof. Triglyphs make up the frieze. It's "friezing in here" sounds like "it's freezing in here" which means it's cold in here. The joke is in using the word "friezing" because the word "frieze" was inserted instead of "freeze". The joke is pulling from architectural terms.
The triglyph commented , ' It ' s friezing in here . .