Risk refers to the degree of variability in investment returns you are willing to bear. It is the chance that an investment's actual return will be different than expected. Every investment carries some level of risk and you must establish your own level of risk tolerance.
An investor with a high risk tolerance is likely to invest in securities such as stocks in startup companies and is willing to accept the possibility that the value of his/her portfolio will decline, at least in the short-term.
An investor with a low risk tolerance, on the other hand, tends to invest predominantly in stable stocks and/or highly-graded bonds. Your risk tolerance is subjective and may vary according to your age, needs, goals, even your personal disposition.