From Wikipedia:
TERRA in biology is an abbreviation for “TElomeric Repeat-containing RNA”. TERRA is RNA that is transcribed from telomeres — the repeating 6-nucleotide sequences that cap the ends of chromosomes. TERRA functions with shelterin to inhibit telomere lengthening by the enzyme telomerase.
TERRAs are essential for telomere length and maintenance. At least four factors contribute to telomere maintenance: telomerase, shelterin, TERRA and the CST Complex.
TERRA can also regulate telomere length by increasing euchromatin formation. On the other hand, nonsense-mediated decay factor enrichment at telomeres may exist to prevent TERRA inhibition of telomerase. TERRA levels vary during the cell cycle, decreasing during S phase, and increasing in the transition from G2 phase to G1 phase.
In Cognimed: TERRA is the hand of the developmental clock. In our model the telomere/TERRA–PRC2 axis is the single clock that times differentiation: each cell fate unlocks when Polycomb repression, withdrawn on this clock, falls below that fate’s threshold. See Cognimed — Cognitive Biology.