Sum of arithmetic sequence, tolerance d = 0.03, according to the general formula an = a1 + (n-1) d, what does an mean?

Sum of arithmetic sequence, tolerance d = 0.03, according to the general formula an = a1 + (n-1) d, what does an mean?

An refers to the nth item in the sequence
a5=a1+4d
a100=a1+99d