The phrase that completes the sentence is subject to more uncertainty.
The complete sentence reads: Long-term bonds are subject to more uncertainty than short-term bonds.
This is equivalent to say that they are more risky. This is kind of intuitive if you think that the further the time to maturity the more the changes in the economy orin the situation of the market will be subject to fluctuations. Also, you can think that the ability to predict how a specific issuer of bonds will be doing in many years is more difficult than to predict the situation in few years.