Risk-on, risk-off (RoRo) investing describes a process where investors move to riskier potentially higher yielding investments and then back again to supposedly lower yielding investments which are perceived to have lower risk.
Quite often risk-on, risk-off behaviour follows global markets, where periods of perceived low financial risk encourage investors to take risk, therefore creating a risk-on situation, and periods of perceived high financial risk cause investors to take less risk, creating a risk-off situation. 
The switching between high risk and low risk investments began to happen more frequently and in greater volumes in the fallout to the 2008/2009 financial crisis.
Roro (risk on, risk off) can cause investors to behave in a herd-like manner depending on the risk environment. This investor behaviour is more likely to occur in times of economic uncertainty. 
At the beginning of 2013 analysts speculated that the era of Risk On, Risk Off might be coming to an end because there were signs that the US Federal Reserve might be leaning towards ending quantitative easing at some point during 2013. Quantitative easing, or QE, by the US and the UK has artificially depressed yields on traditional safe haven investments such as Treasuries and gilts squeezing investors into riskier investment forays.