While transactions from an insider shouldn't be used as the sole item to make an investment or trading decision, an insider buying or selling stock in their company can be a good added factor that leads to more conviction in a decision.
Insiders buying stock after a notable sell off can indicate an insider's long-term belief in the success of the company; insiders buying stock at new highs can be an indication the exec doesn't feel the stock is overvalued.
If the transaction was an open-market transaction, that means that the insider made a concious decision for the company's stock moving forward.
Transaction codes other than P or S are often viewed with less conviction as they are often not tied to a decision by the exec.
Transaction code A indicates the insider may have been forced to sell shares in order to receive compensation the exec was promised upon being hired by a company.