What is the most significant digit of
$$0.00234$$
I have a problem of figuring out where it is $0$ or $2$.
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$\begingroup$Leading zeros are never considered as significant digits, so here for $0.00234$ you have 3 significant digits, 2,3, and 4. The most significant one is 2 (first non-zero from left), because it has the greatest effect on the number (2/1000 has an order of magnitude 10 times larger than 3/10000 and so on...).
Another example: 3.14159
It has six significant digits (all of them give you useful information) and the most significant one is 3.
EDIT to add more detail: $0$s in $0.00234$ are called leading zeros, and such leading zeros are always insignificant. Whereas trailing zeros is the term used for zeros in e.g. $1.2300$ where there's a decimal part, and the zeros are important here as they impose the degree of precision (of measurements e.g.). Last but not least, zeros between significant digits are also considered significant, e.g. $503.103$, which has 6 significant digits.
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