Significant figures (Sig Figs)is an idea that deals with the concept of accuracy in measurement. Physical measurements are inexact. What that means is that the true value of any measured quantity can never be known exactly. For instance, if I am measuring the length of an object, I could always (theoretically at least) use more and more accurate rulers.
Any measurement is an inexact number. When counting sig figs in inexact numbers, the rules in the links below apply. Exact numbers (eg. 12 in a dozen, 9 planets in the solar system, the 1/2 and 2 in KE=1/2mv2) are not measurements, so sig fig rules do not apply to them.
Sig Figs Defined
Determining Sig Figs Rules I HERE
Determining Sig Figs Rules II HERE
Determining Sig Figs Problem Generator HERE
Math with Sig Figs HERE
Rounding Rules HERE
Math with Sig Figs Problem Generator (Mult&Div) HERE
Math with Sig Figs Problem Generator (Add/Sub) HERE