Significant figures (s.f.) show how precisely a value is known. Count from the first non-zero digit on the left. Zeros at the start of a number are not significant, but zeros between or after other digits can be. To round to a chosen number of s.f., look at the next digit. If it is 5 or more, round up, otherwise leave the last figure alone. For example, 0.004567 is 0.0046 to 2 s.f. and 0.00457 to 3 s.f. Counting is the reverse job: 0.0038 has 2 s.f. and 0.00382 has 3 s.f. In a calculation, round only at the end, and give the answer to 2 or 3 s.f. or to the same number as the data you were given.
Standard form writes a number as A × 10n, where A is at least 1 and less than 10, and n is a whole number, so 6.2 is a fine value for A but 62 is not. Large numbers have a positive power of 10 and small numbers have a negative power. For example, 6300000 is 6.3 × 106.
To write 0.00045 in standard form, move the decimal point 4 places to the right to make A = 4.5. Moving it right means the power is negative, so 0.00045 = 4.5 × 10-4. Standard form is handy for physics values such as the speed of light, 3 × 108 m/s.