There are some similarities and if you know Hindi (or Punjabi) you can probably understand the context of what is being spoken in Punjabi (or Hindi). However, you won't be able to understand every word of the other language. Also, the script is not the same. Again, there are some alphabets that are similar but you won't be able to read or write both languages just by knowing the script of one of them.
Just to give a bit of a cultural perspective, in India almost everyone who speaks Punjabi is able to speak Hindi. However, the opposite is not true as Hindi is a much more widely spoken language.