#log# is a function and therefore we need an argument to talk about it's value.
Explanation:
#log(x)# is whatever value #y# which is required to make #10^y = x#
#log(1000)# is #3# since #10^3=1000#
The default base for #log# as seen above is #10#.
Other bases can be specified as: #color(white)("XXX")log_3(x)# which would be the value #y# needed to make #3^y =x# #color(white)("XXX")log_3(81) = 4# since #3^4=81#