What do the letters in the arithmatic formulae mean

The 2 formula is

un = a + (n – 1)d

Sn =1/2n(a + l) =1/2n[2a + (n − 1)d]

What do the letters mean (especially Un)?

un is the value of a term, so the value of the 5th term would be equal to u5
a is the first term in an arithmetic series, so if a series was 2,4,6,8,10.. a would be 2
n is the number of the term you're looking for, so if I wanted to find the 5th term n would be 5
d is the difference in an arithmetic series, so if a series was 3,6,9,12,15.. then d would be 3
Sn is the sum of a series up to a certain point, so say I wanted to find the sum of the first 5 value of a sequence then I would write S5

Lets do some examples

Say a = 4 and d = 3 and I want to find the 51st term:

u51 = 4 + (51 - 1)3
u51 = 4 + 150
u51 = 154

So the value of the 51st term is 154

Say a = 12 and d = 7 and I want to find the 201st term:

u201 = 12 + (201 - 1)7
u201 = 12 + 1400
u201 = 1412

So the value of the 201st term is 1412

Now lets do some examples for the sum of a series, your formula is incorrect by the way; it's supposed to be Sn = 1/2n(2a + (n - 1)d)

Say a = 4 and d = 12 and I want to find the sum of the first 10 values:

S10 = 10/2((2 x 4) + (10-1)12)
S10 = 5(8 + 108)
S10 = 5(116)
S10 = 580

Say a = 7 and d = 9 and I want to find the sum of the first 20 values:

S20 = 20/2((2 x 7) + (20 - 1)9)
S20 = 10(14 + 171)
S20 = 10(185)
S20 = 1850

I hope this helps