Perhaps they're referring to leveraged ETFs like UPRO (Direxion Daily 3X Bull & Bear S&P) and TQQQ (ProShares UltraPro QQQ).

Be advised that they are not designed to be held over more than a day, although with the current bull market, they have not actually been hit by the decay factor inherent to all leveraged ETFs.

Other higher-risk ETFs would be things like solar/wind sector ETFs (e.g. TAN, FAN, PBW, IQCLN, QCLN, etc.) or bleeding-edge biotech (e.g. SBIO).