The #p# function means to take the negative of the log to the base 10. The same for #pH#; here, if #pH# #=# #1#, then #[H^+]# #=# #10^-1# #mol*L^-1#. I recommend that you have a good look at your texts, and get this concept straight.
For instance when we write #log_ab = c#, we are saying that #a^c# #=# #b#. For practice, can you tell me what #log_(10)100#, and #log_(10)1000# are? Years ago, (20-30 years), before the advent of cheap electronic calculators, students used to be issued with log tables for multiplication and division.