The Line 1 Checksum is determined by adding all the previous numbers in Line 1 and taking the last digit in the final sum. All letters, periods and plus signs are taken as "0". Negative signs are taken as "1".

This is mainly used to verify the first line's authenticity and/or its integrity upon receipt.

For the ISS TLE - Line 1:

1+2+5+5+4+4+U(0)+9+8+0+6+7+A(0)+0+6+0+5+2+.(0)+3+4+7+6+7+3+6+1+.(0)+0+0+0+1+3+9+4+9+0+0+0+0+0+-(1)+0+9+7+1+2+7+-(1)+4+0+3+9+3 = 174

Taking the last digit gives a Line 1 Checksum of 4, which matches the final digit in Line 1.

Many orbit propagators do not read or use this value.