It just means that there are no common factors. For example, the LCM of 5 and 7 is 5*7=35 because 5 and 7 have no common factors. But 10 and 14 do have a common factor 2, so the LCM is 2*5*7=70 not 140=10*14. You could say that the LCM of a and b is ab/HCF(a,b). In other words a=hm and b=hn where h is the HCF and m and n are integers. LCM=hm*hn/h=mnh. If a is a multiple of b then we can write LCM=ab/b=a.
To work out the LCM of more than two integers, take a pair and calculate the LCM then take the next integer and calculate the LCM of this integer with the LCM of the first two integers, and so on. So we get LCM(a,b,c)=LCM(LCM(a,b),c), LCM(LCM(a,c),b) or LCM(LCM(b,c),a). For example: 4, 18, 12: LCM(4,18)=4*18/2=36; LCM(36,12)=36*12/12=36. So the LCM of 4, 18 and 12 is 36. Or LCM(18,12)=18*12/6=36; LCM(36,4)=36*4/4=36. Or LCM(4,12)=4*12/4=12; LCM(12,18)=12*18/6=36.